HeyScott, we've got a snitch over here as well called the Separation Monitoring Function. The thing is, no one seems to agree as to what the tolerance is - some people say it is 4.9 miles, others say three and a half whilst I'm sure I heard it said that you have to get aircraft within three miles of each other before it goes off. It also takes into account areas where separation is increased away from the appropriate radar heads which means no 'cheating' over areas of the North Sea.
Anyway, no matter what the distance is, the existance of SMF makes you err on the side of caution and make sure you have every inch, plus a few more for good luck, of those five (or ten where appropriate) nautical miles.