There are no rules. Whatever works best for you is the way to do it.
I Usually bend upwards with my third finger but depending on what note is coming after it, I'll bend downwards.
With the top E string, you're pretty much stuck with doing up bends becauseyou'll just run out of room.
Generally, I'll do both up or down depending on what the lead is and the kind of vibratro I'd like to get.