I make it a point to visit everyone who visits me but I follow those who I actually want to visit again. Good writing is always a good thing. I tend not to follow those who are ONLY product reviews and/or giveaways.
That said, even though I follow a lot of blogs, I don't necessarily feel "obligated" to read each and every post. I run through my Google reader when I have time and read those which stand out at the time.
I just follow everyone. I have a folder in my Google Reader called "Favorites". If someone keeps posting great content that I like (I occassionally skim my other subscriptions), I add them to my favorite folder. Then periodically I go through and delete subscriptions who 1) aren't blogging anymore 2) are only doing reviews/giveaways (and not good ones at that) 3) are just not my "type" lol