Power switch placement
No, there are no conventions. If putting the switch in the wire connecting battery to ground does the job then so be it. Not all battery-powered circuits are grounded and, those that are, may have positive or negative terminals of the battery grounded. I'm mentioning this because the question assumed a negative ground.

If a battery powered circuit has a remote activation switch, quite a few designers would choose to use an NPN BJT or N channel MOSFET in the negative lead of the battery in order to activate it; the reason being that better "on" characteristics are obtainable for the same price.

I see no reason to justify always switching the ungrounded terminal of the battery on the basis that the circuit will "remain live". It's battery powered and like I said earlier, a lot of battery powered equipment is ungrounded.

However, lets explore the problem of switching the relevant battery terminal to ground to activate the circuit - until that switch is closed there can be no more risk of "stray" leakages, currents or voltages partially powering the circuit because the battery will have an open circuited terminal that isn't going anywhere by itself!