Today I laid tile in a 25sq ft bathroom with a concrete subfloor. The concrete had a slight "bow" to it, meaning if you hold a straight edge across the 5' width, there is a gap of about 1/8" in the center. The tiles pretty much follow that slight curve. Will this cause any problems with cracking? If it matters, the tiles are 12" squares, inexpensive ceramic.
Nah. You are good to go. Tiles will crack if they bear weight over an unsupported spot. The bow in itself is irrelevant.
Did you use comb to comb the mortar? The most important is to have the mortar under the whole tile surface. The bow is not a problem then.