what places offer the best incentives for registering with them?

BB&B offers 10% discount after your wedding to get things off your registry, plus if something is on your registry you can get cash back for it.

Also, for your fine china Fortunoff gives you the salad bowl for free when someone purchases a place setting.

Macy's offers 5% cash back for everything anyone purchases off your registry, and 10% cashback on your own purchases - IF you have a Macy's card. Also offers 10% off wedding bands, commemorative jewelry and attendant's jewelry, 10% completion bonus for 90 days after the wedding, and 20% if you buy your gown at the Bridal salon.

They also have a Bridal Bonus Program: When the purchases from your registry reach:

With purchases of 10 place settings of china, you'll get an open vegetable bowl

With 10 place settings of stainless flatware, you get a serving set

With 10 pieces of crystal stemware in same size, you get an 11th.