Of course, it depends on what you're flying.
Most of the time I'll fly final in a crab, and transition to a slip while in the flare (this is in light GA
singles and such). Of course, this is mainly for passenger comfort, there should be no danger in flying in a side slip, provided you don't stall the airplane, of course (or you're not low on fuel or something... not that I've never had the low fuel lights go off in a new skyhawk because of a prolonged forward slip and minimum fuel on board, no, never...) Some planes though, this doesn't work too well in.
The Cub is a good example. When flying her from the back seat, it is useful to fly final in a forward slip, just so you can see where you're landing, or in some cases, to see what kind of wildlife is on your runway. Again, transitioning to a side slip in the flare (or it will get REAL interesting REAL fast).
I've read that many WWII fighters didn't even bother flying final for this reason (not being able to see infront) but flew a descending turn from downwind to flare.