This seems fairly straight-forward. If you can and have access to a printer, buy them online. I've done this with Megabus and Greyhound. Then when you get there there's no last minute queuing (which can be bad sometimes) and you just show your form and get on the bus. Simple!
Generally they ask you to be there 15 minutes before (at least that's Boltbus's rules), but it all depends on how much stress you want to alleviate ;)
One point to note - Boltbus, for example, stops sales of online tickets 3 hours before departure. So if you DO want to buy online, do it promptly.