Primer

As long as the paint is in good condition and no stain issues, you don't need to prime.

There is a lot involved with that question. Generally, if you are doing a mild color change (or simple repaint) on top of a paint of known quality, no you don't have to prime. Priming might be needed for:

Extensive drywall repairs
Topcoating "Builder paint"
Light->Dark color change, especially deep green and red. (Use a grey-tinted primer for these)
Oil paint (use a bonding primer)
Surfaces with past adhesion issues
Difficult environments, such as kitchens and bathrooms

SirWired

Thanks! We decided that we are just gonna prime everything. Easy enough, and will deliver the color we want in the end. Ben