Rick's right. Don't leave the old plugs in the head. And, if you really want to make sure the plugs seal and stay in place, do the same as most NASCAR engine builders do and secure each plug with three brads similar to those that retain the sheet-metal baffle to the bottom of stock SB intake manifolds. This will require drilling three small holes adjacent to each plug flange so the brad head will overlap the flange when it's driven in.
If you're really paranoid, seal and retain the outer edge of the plug by applying a bead of epoxy such as J-B Weld around the edge of the flange. Do this if you're running a 500 mile race and a $200,000 purse is on the line.
Back to using RTV silicon sealer: I've never seen a core plug come out using the procedure I originally described, but being careful can never be criticized.
Cheers,