Depends on how much data the phone is using and what your limits are. Restricting background data might mean getting less frequent email or message notifications if that app isn't in the foreground, or somewhat decreased performance of other apps if they're not allowed to cache data from the web while in the background.
You could always do a trial of restricting background data for a day, and see if it seems to impact your usage at all. If not, and if you need to watch your data, then keep it restricted.