I'm using the arduino ISP sketch, yes.
The ArduinoISP sketch uses SPI to program the target. After disabling RESET you will not
be able to use the ArduinoISP sketch to make any changes to the target including changing the fuses. Unless you have a high voltage programmer, disabling RESET is a one way path. If you first install a bootloader, you will still be able to upload sketches but you will not be able to make any other changes to the target.
In other words, if you did not first install an actual bootloader the target will be "bricked" until you have access to a high voltage programmer.
When I talk about bootloader, I mean the "burn bootloader" command on the arduino ide, before uploading any sketch to the tiny.
You will have to install an actual bootloader before disabling RESET. I suggest you first try installing and using a bootloader before taking that final step of disabling RESET.