Further to PMK, any exposed metal bits, if they are connected to any part of the supply, will be connected to battery -ve. As an accidental grounding of a live wire is most likely to find one of these, the protection needs to be in the supply, i.e. +ve.
If the installation consists of several parts, having the -ve common and fusing the distribution makes sense. If it just a battery, receiver, ESC and servo, it still makes sense to put the fuse where it is conventionally expected, in the +ve side. This after 30 odd years in an industry where equipment was run on -50volts, and all the fuses were in the -ve supply, obviously apart from the +50volt supply........