Should I use Optimized Battery Charging if I don't charge my iPhone overnight?

Not really, Optimized Battery Charging works best when you have it on the charger for the same extended period of time daily, which in most cases is overnight, as it learns from your habits. Otherwise it may not activate all the time if your charging habits are variable. But if you do consider Optimized Battery Charging, you should charge overnight. Not to mention that to activate Optimized Battery Charging, it needs to be charged 9 times for 5 hours or more, so if you don't do that overnight you'll have to find time to do that which may never happen.

My User Tip explains everything you need to know and use for Optimized Battery Charging, including comparing it to Charge Limits, another form of charge habits. Charge Limit or Optimized Battery Chargin… - Apple Community

I would not use optimized charging if you want the fastest possible charge. If not then use optimized charging.

Personally, whether using optimized charging or not, I've never noticed any difference in battery life on any of my devices. My device usage varies from day to day so I just charge them to 100 whenever I feel it's warranted.

I personally do not. I have my battery charged to 100% in the late evening and then turn my phone off (power down) while asleep.