We didn't tell our wedding party they were getting gifts until the day of the wedding when they received them. The MOHs received perfume sampler and their choice to get from shoppers. The BM (Best men) received Calvin Klein cologne sets.
We gave out MOH and BM a perfume sampler so I gave MOH hers the night before when we were setting up. She could still use the samples that came in the box and she can redeem the coupon later for her favourite choice. I was happy to learn she had just ran out of her perfume and was really looking forward to getting the new bottle. It was a perfect gift for her.
If you're planning on giving them something for the wedding, like jewelry, I would give them a heads up. You wouldn't want your team to come all decked out and then either needing to find a secure place to store their own stuff to wear yours, or not wear yours because they prepared their own not knowing you had something planned.
We surprised our bridal party. We're actually doing it this weekend. My fiance is giving his best man his younger brother a watch that he can wear the day of the wedding and I'm giving my MOH a overnight bag that says I do crew and tickets to the backstreet boys at the end of August