Scan once, according to my projected real need, plus some for safety. I only print to 11x14, and my printer handles 13x19, though I never print that large. Once in a while I might make something larger for fun, but I'm not impressed by large prints, so I don't need a lot of res. Most of my film these days is 8x10, with a tiny amount of 5x7 and even less 4x5.
I scan once, at 1200, which will let me make a pretty big print if I want. Then I do all the processing I can in Photoshop layers, leaving the original untouched underneath, and save as a layered TIFF. I have been saving the final file in the cloud, on two different services, and make a jpg at full resolution for Flickr, as a last-shot backup. The cloud files are mirrored on all of the three computers I regularly use. Every once in a while I archive on a hard drive, but I don't trust that. I don't trust DVDs or CDs, having been burned by those in the past (I've been using digital since around 1998 or before, and lost most of that early stuff). I'm more concerned with the backup than the scanning res. That's the reason for TIFFs, too, though that seems to be a contentious issue.* Of course, the original negs will probably always be there.
*[contentious: you bet, there are people who will vehemently disagree with everything everyone else says, and I no long play that game with them. You know who you are; most of you are on my ignore list here, so I won't be seeing your response. Couldn't resist saying that :-) . . .Just saying what I do.....]
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