There is no one shoe fits all solution here. It fully depends on the situation you are facing!
So, e.g.:
You need to destroy an enemy army in one shot. Only the 100% HP bomber stack gets that done -> You send that
There is a large concentration of enemy troops that you need to slowly whittle down, but doing it quickly isn't a priority -> You stack all your bombers to increase their total HP and spread the damage they take evenly
There's a single infantry runner unit that you want to quickly and easily prevent from capturing one of your vulnerable provinces -> you split the low hp bombers so you only need to send the amount of units needed to kill the runner -> keeping your other units flexible for other taskings in the meantime.
I hope that helps explain some of the thoughts/considerations that go into these decisions. And always remember, you can see the current, up to date damage values by heading to the army info window. This let's you make quick and informed decisions on whether a fight is worth taking or you should send more backup.
