Mesh under shirt - always
I spend several hours/day on the bike the training in needed to be competitive. Aside from a clean kit every day I reach for my mesh under layer shirt. A good mesh shirt is indispensible as it raising your cycling shirt up off the skin creating a air pocket that allows evaporation to occur. This in turn keeps you cool when it hot, and warmer when its a bit cool out. This Castelli mesh is my go pieces during the summer months and is built to last with double seams and strong thread construction.
I have been wearing an undershirt for years and cannot imagine not have this essential layer on under my kit to keep the air flowing over my skin.
Get yourself a couple and you will wonder how you lived with out them.