Depends how you plan to scan it.

With a drumscanner or an Imacon it's no problem to get it flat even if originals are extremely curly. Other scanners may have difficulties if they don't have special glass carriers, or wet-mount carriers.

One trick I've used in the past is to re-roll them "inside-out" (emulsion-side out), but don't make too tight roll, then fix it with some "moneydash" rubber - be careful you can twist the emulsion too much (I've went to extremes before by making the roll too small and damaged my emulsion on some frames). Let it stay in this mildly loose roll for a day and they're flat after that.

Normally I just cut them up and let them sit in my (almost-full) negative pocket for a few days and they're pretty much flat after that, enough for me to start working with them conveniently through my drum scanner. Sometimes I rewash and use wetting agent aided drying again only if the originals are extremely dirty, they'll still curl up after drying so there's no help from re-washing in terms of curling.