Rolen, trust me I know who's favored on the Red Sox, I grew up as a Red Sox fan and I'm still a Sox fan (just in exile away from New England), essentially anyone not named Bob Stanley or Carl Everett have gotten their share of cheers over the years.
However for game purposes, which of those players sells the most tickets... are you more likely to pay to go see Trot Nixon vs. Kansas City or Pedro Martinez? In the Boston case I would make Martinez the fan favorite. (Remember even Manny started to hear some boos towards the end of last season).
If you make too many players "fan favorites" then any positive effects have to be balanced by a much larger group of negative players, and quite frankly in My Opinion a rating like that (or the clutch
rating or leadership rating, etc.) must be fairly rare to make it a special bonus to have one of those guys on your squad.
Keep in mind some teams might have more than one of those players, while someone like Montreal (sorry Quebecois) may have no "fan favorite" players, I would just use about 1 per team as the threshold for how many to assign.