Crystal precision is usually 100 PPM by commercial standards. I've used oscillators specified at 20 ppm, but it's seldom worth the extra cost.

What I meant by "precise" is that an instruction takes a precisely defined interval that fits a given task. Now, sometimes it doesn't matter what the rate is, and sometimes it matters very much. If you're interfacing a tape drive, for example, where the filters are designed for a 48 microsecond period, it's best to attempt to use a 48 microsecond period and not something else. Often it makes the difference between counting, say, 21 microseconds, as 21 instruction cycles in a timed loop, as opposed to counting 21 seven times, then counting 20 once or twice, and then going back to 21, as the overhead can be punishing.

RE