Opportunity is missed by most people because it is dressed in overalls and looks like work. - Thomas Edison
Sometimes I will make a highlight white for white shirts, but never done a white base on white unless it was printing on multiple colored garments.
The only reason to do an underbase, be it white ink or clear, would be to assist in the "dot gain" of your inks and to increase how smooth your blends will be. Also to add weight/smoothness to realllly crappy open weave fabric.....When printing directly on a white shirt, your inks will more readily hold their shape/go right into the shirt. This "can" make your blends a little rougher. Most compensate for this in their seps. Adding more saturation of ink in areas they wouldn't normally if printing a base screen. So, as was stated earlier; TLDR: Its artwork dependant.