The reset line has an internal pull-up resistor, but if the environment is noisy it can be insufficient and reset can therefore occur sporadically. Refer to datasheet for value of pull-up resistor on specific devices. Connecting the RESET so that it is possible to enter both high-voltage programming and ordinary low level reset can be achieved by applying a pull-up resistor to the RESET line. This pull-up resistor makes sure that reset does not go low unintended. The pull-up resistor can in theory be of any size, but if the Atmel AVR should be programmed from e.g. STK500/AVRISP the pull-up should not be so strong that the programmer cannot activate RESET by draw the RESET line low. The recommended pull-up resistor is 4.7k Ω or larger when using STK500 for programming. For debugWIRE to function properly, the pull-up must not be smaller than 10k Ω
Personally I've never used a pull up on my programmer boards used with ArduinoISP. If your USB ports are that noisy you've got worse problems than reset pullups.