Might be true for *most* machines, but there have been many many machines that were emptiable, no matter what time you caught them...Pure and simple there is no "pros" technique i believe there are glitches in certain machines that you can "trick" ie the old King kebeb barcode 321 follow me "trick" but as most of these machines it depends how much has been put in before and catching it at the right time.
Any machine that,thanks to programming errors, has 'unaccounted' wins or features can be emptied, provided the win/feature is easily obtainable during normal play.
A fairly obvious recent example would be the superhold error (or was it deliberate? ) that made it possible to get jackpot every time the superhold feature or bonus was acheived on certain Bellfruits (some of which are emulated)
A slightly older 'trick' - one that can also be tested out on an emulated machine - is the free win on 'cash duel' featue on the old ACE 'Wild West' machine.
A bit tedious to perform, but it's interesting to have confirmation that emptiers do/did work (for those who doubt they exist[ed]).