You don't think 'any less' of them but when there's no lines it makes you think 'more about' them I suppose. If they're, comando, on a treadmill by the side of you... you start to get delicious thoughts ;) hahahaha Even a sneaky bit of camel toe (only a bit, nothing overly camelish) is quite nice, a bit of a turn-on... you feel like you're being teased and a lot of the girls know this :)
it depends on if you're trying to attract attention to your glutes or not. A visible panty line will attract more attention, because it gives guys a reason to look at your butt - panty lines stand out pretty noticibly.
You can do what you want- thong, g-string, nothing, or full panties. I'm usually not staring that hard at other people's butts enough to really judge them for panty lines. Even if one was super obvious, its nothing to be embarrassed over.