It is kind of important to learn the spawn sometimes, but there's only a few points its important. Some of it just comes from playing a map over and over again. For instance, in Urban Chaos around the late 30s or early 40s in the combo after dogs and firemen zombies start appearing, you want to head up the bridge as the next few zombies spawn (BSAA solders that turn into bloodshots if you don't get a direct headshot) spawn up on the bridge at that point, and after that the rest of the match is a pretty steady flow of zombies that start from the back of the arena (next to the arcade) and eventually makes its way back to the front.
I was surprised when I got my first 150 combo. I was playing with a good friend of mine, but he and I aren't the best at Mercs (not bad, but not great either) and this was one of his first times playing RE6 Mercs, we just both did it at Leon as we were attempting to unlock the costumes and we managed to do a 150 combo and got through some tricky situations. I was surprised as was he as we both weren't really being serious about it, making a lot of jokes and such, but then both he and I were being good with keeping some enemies alive and spacing kills while going for a lot of melee kills as he and I did always used to go both as BArry and essentially do that in Mercenaries Reunion.