Reasonable temps, and I wear a sleeveless tee under whatever top shirt I have. At least. Cold weather, I wear thermals...
For warmer weather though, I do a lot with the dry-fit stuff - super lightweight, moisture-wicking, super-stretchy, and I'm actually cooler in those than I am with just the top shirt on. I've also found that tucking the dri-fit on the outside of my XDs, and wearing a loose tee on top, actually helps reduce printing because the dri-fit is stretchy and helps blend in the lines of the pistol.
Yes, my Alien holster is very comfortable against bare skin. And if I know I'm sweaty when I get home and unhook, I make sure to field-strip the pistol, dry it, let it air for a few minutes, and put a drop or two of slide lube on the tracks before reassembly. I also never store the pistol in my carry holster.