The protocol is to put the fuse in the +ve, but in practice it makes little difference. A club 500 will pull 28 amps for a split second as the motor starts, but settles down to around 12 amps once flat out & will pull 88 amps if you manage to stall it, so a fuse is a really good idea. Most I have seen us a 25 amp fuse to combat the initial surge, safe in the knowledge that a stalled motor will easily blow a 25 amp fuse, but make sure the fuse is rated below the ESC rating.
Keep an eye on the fuse as it will eventually blow due to the constant initial surges from start, make sure you use decent sized wire & very good connections so as to minimise the power losses through the whole electrical system.
Glenn