Priming is a good idea, especially if you don't know what type of paint originally was used. Be aware that "Self Priming" is over itself or same type of paint. Every manufacture recommends primer when in doubt.
At the vary least you will need to spot prime any raw areas after prepping. If your house is in bad shape and requires a lot of priming, go ahead and prime everything. With a complete prime coat you know your house won't need repainting any time soon. Skipping the priming gives you a 50-50 chance of failure.