do I have to match the db level on all speakers?

If all your speakers but one are at 88db, why not adjust the center -3db? Bear in mind that the acoustics of the room also enter the picture, as does the listening position relative to the speaker positions. You should adjust the speakers via the setup menu so the sound is right for you from your normal viewing/listening position -- the relative speaker efficiencies will then be handled as will all the other factors. Good luck.

I am pretty much on equal distances from each of the speakers (back speakers are closer to the listening position by aprox 2 feet). So I think ideally they all should be at 88db (or at 91db).

Just run the test tone on your HT rcvr/processor and set it by ear.