Speaking as someone who used to serve, I'd prefer if you filled out the tip line yourself. Since I had to add up the tip lines at the end of the night to calculate tips, if the tip line wasn't filled out then I'd have to do all the math myself and then fill it in.
It's not a huge deal, but it doesn't actually make anything easier.
Also, we didn't have to do the math on cash tips. The reconciliation at the end of the night just tells you how much you owe back to the restaurant, or the other way around if you have more tips than cash you're carrying. Cash orders just get rung out to exact change in the computer and you hold onto the difference until the end of the night, or they get change and leave a tip out of that and you don't need to calculate that either.
Maybe that reads confusing. Cash handling for servers is sort of inherently confusing. To answer your question, filling out the tip lines makes it easier on your server.
But I can't emphasize this enough: make sure you get the math right! If you can't get the math right just leave a final total, because any kind of question about the tip means that your server gets the smallest possible tip.
Say you have a $40 order and you write $6 on the tip line, then write $47 on the total line. The managers are going to insist that you only get the $6 because the customer who's paying gets the benefit of the doubt, even if you actually meant to leave a $7 tip. If we catch that before you leave we can ask you to fix the math and initial it, but if you're already gone there's nobody to ask and the server loses that dollar.