I prewash anything I am going to launder in that manner. No exceptions.
I NEVER prewash wools, silks or linens that I plan on dry cleaning. The hand of these fabrics always changes with water laundering. I bought silk matka on sale once, $2/yd, brought it on and laid it on the bed to start dinner. Two hours later the cat had thrown up on the folded folded fabric. Decided to wash all 5 yards, wish I hadn't. Don't care for the soft, fuzzy, limp hand it now has.
Some people like the felting effect these fabrics suffer in water - just not my preference. Especially not a good choice for tailored items.
Remember, RTW is not pre-washed (except for some denim). The crispness of a new item diminishes as it is laundered.