There are several problems with practise sim sessions, not least being which SOPs are you going to be working to? If you are anticipating a sim assessment with Easyjet, it would be crazy to do a practise session with a Ryanair instructor, because their SOPs will be significantly different - even down to what speeds are recommended for different phases of flight.
As has been said, if you are not qualified on type (and even if you are) you will be assessed on how quickly you learn when you do your sim assessment. While I think it may be worthwhile to have a go in an airliner simulator if you've never seen one before, just to ease the nerves and negate the unfamiliarity factor, you are unlikely to gain as much as you'd like to think you will - as Northern Highflyer has illustrated. And, for goodness' sake, don't tell the assessor you were in the sim practising the night before!
If you've recently done your MCC and thus have a fair idea of what goes on on the flightdeck, a practise sim is probably a waste of money. If it's been a while, or you really think your nerves will be shot if you don't do it, then go ahead.
Scroggs