if the timer ISR is active the user interrupt has to wait for the timer interrupt ISR to finish
Since I call a single iteration of Timer1 in this interrupt, I know that Timer1 won't be busy. As far as Timer0 goes, I don't think I would have any code beyond the Ethernet polling that would be operating that would use it.
My application is film scanning. The delay between the incoming pulse on INT0 affects the vertical framing of each frame, so consisten latency is kind of important.