My own approach, generally, is to review whatever I feel like playing at the time. If I can get a review key, I won't bother unless it's a game I actually would buy. And of course, I do review a lot of games I buy after never having been sent a key. I don't think there's much point in covering a game you weren't actually interested in playing, and some people only find time for games others have told them are masterpieces. Considering all the entertainment choices in the world, I guess it's only natural to elect to spend one's time with proven quantities. Of course, then you start to worry that you're missing out on something great, and when you do that, the world benefits as you work either to dispel that notion (in the case of pickhut's The Legend of Kage 2 review here), or to alert people to another awesome experience.
"Too often we enjoy the comfort of opinion without the discomfort of thought." - John F. Kennedy on reality
"What if everything you see is more than what you see--the person next to you is a warrior and the space that appears empty is a secret door to another world? What if something appears that shouldn't? You either dismiss it, or you accept that there is much more to the world than you think. Perhaps it really is a doorway, and if you choose to go inside, you'll find many unexpected things." - Shigeru Miyamoto on secret doors to another world