Using level ups against bosses ("ding fighting?") will optimally include a bit of regen fighting. The idea is that after the combat exchange with the boss, it's rare that you're totally tapped out. If the boss is doing 40% of your health per blow, and you are at 30%, you should explore a few tiles until you're at 41% (or so) before finishing the level up. Similarly, you should avoid dinging if you are just one mana shy of casting an additional fireball. etc.
There are some exceptions to this, but almost no exceptions to the idea that you should fully exhaust yourself before level ups. Sometimes, it is best to use your leftover resources to 'prep' mid-level monsters that would otherwise would not be one shot wonders. The deciding factor here is whether is not the extra experience you would gain from this would make the difference between an additional levelup or not.