If there's a few here an there you can cut the tab portion of the shingle off.
I have also done 'three times' full tab cuts, meaning the majority of the tabs had begun curling thus we cut every single tab on the roof off before laying over with the new.
Nailing curled tabs down seems like a bad idea to me,
(don't know this for fact because I have never done it)
the curled portion will most likely crack as you nail into it and end up leaving pieces "Small" but pieces just the same of loose shingles laying between the existing and the new and when the shingles move 'natural course of expanding/contracting' those loose pieces are going to move also and very well may cause issues.