Typically timing yourself using a stopwatch or GPS is going to be far from as accurate as the timing used in an official race. Unless you're running tens of seconds faster than your race results in practice when timing yourself, I think there is too big of a margin of error to say you've PR'd in a non-race setting. Sure, if your best official race mile was 5:00 and you're doing 4:30 in practice with a stopwatch, the margin of error is not going to be an issue. You're definitely a sub 5-miler. But if you're running 4:55 in practice with coach's stopwatch, you're not really a sub 5 miler.