Well, I have no strict method (at least I didn`t analyze it), but I generally have 2 types of skill: Nice to have and "Defining" Skills.
The Nice to Have skills I usually raise not above 5 ranks (hey, this can give you some synergy bonus, and it usually is still a easy computable number

), expect I have found the skill especially useful or have some extra skill points to use.
The defining Skills of the characters are get maxed out.
For a Bard, this would be Perform and Gather Information, plus some different other skills. Oh, and never forget Use Magic Device

For a Rogue, that works as a Thief, search, disable Device and so on are maxed.
For a Rogue that would be something like a negotiater or spy I would max out the Charisma Based Skills and Sense Motive.
For a Rogue as a thug I would max out Initmidate and the combat related skills like Tumble.
For a Fighter, Ride is one of the more useful, maybe I would try to spend some cross class skills on Diplomacy (I did this at least once to get rid of the negative Charisma Modifier on these checks) and some ranks in Craft Skills.
And so on and so forth. I think you get it.
