None of my nucs have SBB, for the reason given in the last two posts. You normally do not have mite issues till later in the year, well past any nuc building period. In building nucs, you are many times raising queens, putting the bees through broodless cycles, etc. Mites are handled by good genetics and if you do have mite issues, it will be from August on. Not in April, May, June, which is nuc building time for most.