Need opinion: what's worth maxing?

Obviously, it's not worth making a Great Horned Warrior + and maxing him. But at what point (grade & bp) do you generally think it's worth it to try to max a monster? Is it worth it to max a 15 bp A+? I realize there will be exceptions (e.g. Child of Pit?), but I'm looking for general guidance. My guess is that I'll naturally want to stop leveling some because by the time I get to level 50 or so, I'll have at least 5 monsters with higher bp. Is that what happens? And when?

FWIW, I'm a newb (level ~30).

I was in this same boat when I started. Since you're only lvl 30 don't worry too much about leveling up and battling. You need to just work on trading for a good formation. Get 5 cards that are the same guild/species/rarity then start to level them. A good starting point would be to to go this site:
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then click on the A+ monsters in your guild and sort that list by species. That wiki page isn't always accurate, but I find most of the monster descriptions to be close.

The other nice thing if you pick a guild and species is you can then slowly upgrade later. So if you pick Psycho brutes, you could start with WWE and others, and slowly phase them out for the stronger cards and still have your bonuses going. Much easier than going back and forth between types of team just because it's what you can afford at the time.