I'd try it on if I'm unsure if it would fit well or if I don't know if it will look good on me, but sometimes I could already tell just by looking at it and in those cases I'll usually just purchase it and go so as not to waste too much time. I also do this a lot more when the item I'm buying isn't too expensive. I'm a lot more careful with my purchases if they are pricey.
It's kind of a dumb thing not to try on clothes before you purchase them but you'd be surprised how many people just get clothes off the rack and pay for it and hope for the best. As for me, it depends on what type of clothing I'm buying. Usually, for things like a simple shirt, I just take a look at it for while, check on it in front of the mirror and pay for it. As for pants, I always try them out first, there's nothing more frustrating than having to pay $25 for a pair of pants that doesn't fit you properly.
I dislike trying on clothes, but usually, I have to, just because of the way I'm shaped. Just because the jeans say they'll fit me, doesn't mean I'll actually be able to get them up past my thighs (sadly, I actually had this happen.) If I know it will fit (if I have something exactly the same at home, for example) I won't bother with it.
