You can, as long as thay don't interfere with the ICSP or you're projects breaks if the pin is driven during ICSP. For example, a button to GND used with internal pull up is fine (as long as it's not pressed during programming) but a switch or sensor output (aka the pin is externally driven) it will screw with ICSP. And driving a led (with low current*) is fine but if that pin drives the launch or a nuclear missile you might not want to do that because it will get triggered during programming. 
*Because the ATtiny and programmer (depending on it's MISO or MOSI) needs to be able to drive it while programming as well.