I work designing ISP dongles for ISP programmers for commercial ICT fixtures. In many commercial products, you do need to take care that while you're programming one microcontroller, there is no possibility of turning on other microcontrollers in the circuit. It definitely doesn't help if you use the same reset node between multiple microcontrollers. Many of the products we deliver, we add relays to hold other chips in reset while we are programming one of the microcontrollers on the product. If the reset pins for all microcontrollers include a resistive pull-up resistor to VCC, so that you can safely tie the reset pin down for those microcontrollers to hold them in reset while you program other parts.
In general, after designing an ISP solution, you typically try to avoid any of the microcontrollers on the product from executing code, even the device you're programming. I've found that this could be pretty tricky at times, especially with PIC microcontrollers (PIC12LF1824, for instance) due to the high-voltage requirement for program mode entry and the fact that the reset pin is configurable as an input/output pin -- in which case, it's impossible to prevent those chips from turning on. In that case, you want a way to isolate the programming signals so activity on other chips does not interfere with your ISP operation.