If a prestige class requires 5 ranks in a skill and I have 6 ranks in that skill, do I still meet the requirements?

Yes, because skill ranks are inclusive: if you have 6 ranks in a skill, then you have 5 ranks in that skill, and therefore meet the "have 5 ranks in [this] skill" requirement.

In the same way, if you have a BAB of +6, then you have a BAB of +5, and therefore meet the "have BAB +5" requirement.

In the same way, if you have Str 15, then you have Str 13, and therefore meet the "Str 13" feat prerequisite for Power Attack.

However, spellcasting ability is not inclusive: it is possible (mainly through the use of spell-like abilities) to be able to cast 3rd-level spells but not 2nd-level spells. If you can only cast 3rd-level spells, that does not meet the requirement of "able to cast 2nd-level spells."

Likewise, feat prerequisites are not inclusive, as it is possible for a creature to have a feat without meeting that feat's prerequisites. For example, a ranger can select Precise Shot as a ranger bonus feat without having the Point Blank Shot feat; he does not meet the prerequisites for Far Shot (which has Point Blank Shot as a prerequisite) because he doesn't actually have the Point Blank Shot feat, even though he has a feat that lists Point Blank Shot as a prerequisite.

[Source]