I'm dealing with relatively small motors in my project (it's a mini tabletop Skee-Ball type thing) akin to those used in a small robot. They draw about 0.15 A normally and run in short bursts (ball serve, prize dispense) but as motors they are inductive loads (with the necessary flyback diodes) and also will briefly draw stall current (0.6 A) when first starting.

I will have capacitors next to the motor control chips.

I have LEDs pointed at photo-resistors to detect balls, coins going in and the prize coming out.

I do have electronic sounds for the game, so I would assume that they shouldn't share a ground with the motors, for example.

Since the grounds obviously must come together at some point to complete the circuit:, my main question is where should they come together? This isn't all on one board, it will be several separate ones.

I probably won't have a metal case and even if I did I wouldn't use that as a primary ground (though I would definitely have a safety ground if there was any kind of high voltage within the case. My project will run entirely on 9V DC, with the choice of using an external power brick or 6 AA batteries. Speaking of safety, I do have a fuse on the input)