Here's the way I do it:
if I'm scrolling through Latest, I try to give a lot of upvotes (I don't upvote anonymous memes, memes that insult other users, or memes that break submission guidelines, but if I understand it and it's even half-funny I figure they get an upvote for trying).
if I'm scrolling through Hot, I use upvotes and downvotes to "steer" the memes in the direction I want them to go. I upvote it if I want to see that meme move closer to the Front page, and I downvote it if I want it to move away from the Front page.
The reason I do that is because the Front page is the part of imgflip that's most visible to the rest of the world, so I want to do my part to make sure that the memes that get there are the best of what we've been making on the site.
Of course, how everyone uses the upvote and downvote button is up to them. :)