jumbojetset
It comes down to personal choice. If you have not flown say a jet sim before and have a sim assessment in a jet sim coming up then unless you are Chuck Yaeger then practicing in something similar will be of benefit to you. If it is a big step up for you then the first thing you will notice is just how quickly everything happens, how sensitive it is in pitch, how the nose comes up when you put power on (and vice versa) and how you need handfuls of thrust lever to make comparitively small changes in speed.
What you have to remember is in a sim assessment they are looking for improvement, yes, but they are also looking for a reasonable level to start with. Better to start off (comparatively) well and improve on that than make a pigs ear of your first departure/ ILS and then just about scrape a reasonable one together on subsequent attempts.
Practice will get you used to making finer adjustments and having a better 'ballpark' figure to aim at to achieve say straight and level, a level off in descent or rotation rate etc etc. Yes, sims are expensive, but look at the number of guys who are currently getting rejection letters from Britannia, many due to below par sim performance. In the big scheme of things a few hundred quid more could have potentially put them on the ladder for their airline career, now perhaps they may have to wait months or heaven forbid a year or two to secure similar employment.
A lot of it comes down to how much you want it.
Best of luck.
PP