We're all lip kissers here (actually, we're everywhere kissers - forehead, noses, bellybuttons, toes, legs and elbows - you get the idea).
I kiss a lot of family members on the lips - but now that I think about it, there's some I do, and some I don't. Like my grandpa - he gets kisses on the cheek. Just the way it is - I don't even think about it, it's just the way it happens. My kids kiss just about everyone
on the lips. Some people kind of hesitate, but my boys (3 and 5) can be pretty persistent.
My dad doesn't like lip kisses, though, and always kinda ducks out of them from the boys (the oldest will look kinda confused for a minute then, okay yea this is grandpa - kiss on cheek). I've told them simply grandpa prefers cheek kisses. No biggie - they're not traumatized or anything, and it doesn't seem to affect their confidence or feelings about kissing everyone else.
And I DO agree - my kids learn to set boundaries (and let other people know them) by understanding other people have boundaries and learning to respect them. It's totally okay (and has happened) where someone wanted hugs or something from my boys and they've said, "No thanks, I don't want to ____ right now"; "I don't feel like it"; "No, that makes me uncomfortable so I don't want to" (*older boy); an even once, "No hug, but you can shake my hand".