I always use 14 for lights. I made the mistake once of using 12, since all existing lighting was done in 12. 12 crowds a box quickly & even when in limits, makes getting switches lined up a pain. Customer wants a dimmer, have to pack wire in with a hammer handle.
I use 12 for kitchen & bath ckts, as required. Sometimes for other recep ckts, like a workshop with lots of tools. If I kow things will be redone a lot, I use deep metal boxes with RS covers, allows more space to tap for a future something.
I have seen a space heater overload a 15 amp ckt by itself, a few of the larger ones. If I know space heaters will be used, I go to 12 for sure.
Sometimes I put 2 br's together on 12 instead of separate on 14. I play by ear, see how crowded panel is, how much load may be used, what I have on the truck, whether yellow or white cable is more appealing that day.......