I do have a small dislike of IGN because my gaming opinions and views don't align with them very often, and I think they are very pompous about what they think is good or bad (made evident in their blogs). I will from time to time read up on what they have to say but I don't take any score they publish to heart becaue quite frankly I do enough research on every game I purchase that by the time it comes out, without reading a single review I know I want it.
On the other hand, I have a huuuuuuuuuuuge hatred for gamespot. They are Sony fanboys (which is the worst kind of fanboy). You have all different editors reviewing games on every system, and half of the editors on the site really don't like Nintendo games. When games are reviewed you see terrible scores for Nintendo games and great scores for mediocre crap games on the PS2, hell even the Xbox gets the shaft over there. Games that I loved from the N64 era were sh!t all over by Jeff Gerstmann (who I especially hate), then games that I could never see the value in were praised (without reason). When most of those reviewers can't get into a game (usually because they suck at it) they rip on it. They find all the negatives and bring those to the forefront in their review like they are 2 year old or something.
As far a set score for all games out there, for each of us there is a set score out there for every single game. We all have different opinions and tastes. Though all of us in these forums are Nintendo fans, we still aren't guaranteed to like the same thing (see Smash Bros. vs. Melee debate). But in a broader sense, there is a more fitting score for all games out there than the score it receives typically when it is really high, or really low(this is more rare).
Bottom line: Read the review for information about the game, if you like what they have to say or don't like what they have to say, keep the score but don't take it to heart. You know what you like and dislike, if your likes line up with the focus of the game you should be just fine. I usually read the impressions for a much more solid description of what's going on, though they all seem to wind up sounding very positive.