You probably not going to loos too much accuracy at all.
A normal relay has a 5 msec change overtime, if you are moving 2"/sec (120"/min) this translates to an extra 10 thou of an inch overshoot. Normally Mach3 is configured to immediately stop all drives and then slowly back out, if the back out rate is something like 3"/min (1/20" per sec) then the error is in the sub 1 thou of an inch range.
David Campbell