Get Up if You Can, Call if You Can't
I'd use your personal judgement on this one. What I do on long-haul flights is get up and go ask myself. For starters it makes me get up and move around a bit which is not at all bad. Secondly, I tell myself that flight attendants might be busy with something else and that my getting up might actually be helpful for them. From the way you worded your question I'm getting the idea that you think the same.
Now all this is all nice and dandy, if you are in a position to easily get up and move around. In other words, this works if you are seated on the aisle. However, if you are seated next to the window, with two other seats separating you from the aisle, you might want to press the button and wait. Calling the flight attendant might be much less disturbance than having to play musical chairs with two other passengers twice (to get out and back in) to go pick up a beer. Moreover, once you call the flight attendant, you can ask for two beers, so that you won't have to repeat the process again soon thereafter.
Now, all of this is just my personal way of going about this. For completeness sake I managed to find some interesting posts here and there on the web offering similar and different point of views.
Get Up in Economy, Call in Business/First
This post on OneMileAtATime makes a somewhat valid point in distinguishing between economy class and business. In economy the attendant-to-passenger ratio is on the understaffed side of the scale (some 1:50 average). If many passengers push the button at the same time, the aircraft staff are likely to go crazy trying to manage all the requests. The same can also be said if many passengers get up concurrently to go towards the galley. However, on most long-haul flights you'll find a nice self-serve tray with drinks and snacks set-up in the galley from which you can quench most of your cravings. If beer isn't on the tray, you can go ask the staff directly.
In business/first class on the other hand the attendant-to-passenger ratio is much lower, and staff generally have a much easier time dealing with passenger requests. In that case you could opt for pressing the button over getting up yourself if you wish to do so.
Be Considerate to Staff Privacy
The post goes on to mention another interesting scenario. The author says that galleys are usually screened away from the cabin with curtains, aimed ad providing staff with some kind of privacy/relaxed environment. There's a chance that airline policy or cultural norm might dictate that passengers aren't supposed to see the staff in their relaxed mode. Hence, by walking up to the galley yourself you might be somewhat causing an awkward situation.
Quoting directly from the linked site:
On non-US airlines I think it’s always acceptable to push the call button for something service related. And I’ll take it a step further. Aside from airlines that have onboard bars, I find most crews actually prefer that you push the call button rather than coming to the galley, at least between meals.
What you might notice on most international carriers is that they have thick curtains between the galley and the rest of the cabin, so most of the time between meals the crews will draw the “curtains” around the galley. I’ve walked in on some really awkward situations while thinking I was doing them a favor by going to the galley to request something rather than pushing the call button.
Now if I’m going to the lavatory and see a crew member in sight then I might ask them for something, but I’ve found in the galley they’re often eating, applying makeup, or doing other less kosher things. So by pushing the call button you’re letting them present themselves to you as they’d like to be seen rather than basically opening their “curtain” uninvited.
Be Polite
As an added bonus, whatever you do keep a smile on your face and be polite. I'm prepared to bet that being arrogant and rude to the staff is much more of a problem than you pressing the button once when you're not supposed to.
In the words of this flight attendant on FlyerTalk puts it:
As a philosophy, I am happy for you to use it at your discretion – as long as your discretion doesn’t mistake “flight attendant” for “personal servant” (that definitely happens!), and remember (if I may reiterate) that there are usually 200 hundred of you onboard. If you respect those facts and still see a need to ring, I support you. Most of my friends, I think, feel the same. Let’s be honest among friends though: In reality you just have to feel out your crew.
I will put out there that drink requests are a little bit special. I do prefer you come ask for it yourself (mostly I’m just afraid if passengers see me “running” for a drink, 1000 other dings will follow and it will get out of hand), but I wouldn’t be grumpy about it. Not unless you start dinging over and over.
As I like to say, all things in moderation. Trapped at the window? I can see that’s awkward! Need a hand? I’ll happily help. Feel the need for a cheeky drink that you’re occasionally too lazy to come to the galley for? I might tease you, but oh, go on then!
Still unsure? Here’s another measure: If you’re the kind of passenger to care if you should use it, you’re probably ok to do so. I promise to do it with pleasure, if you promise not to get carried away.
