Is it normal for a login to the webstore to fire the recalc function from the scriptable cart?

I can't say conclusively but it would make sense if it did. Loading a user's record data checks their saved cart and updates it accordingly. So it will definitely do it if they have an item in their cart before login, or if there are items in their saved cart. If they're both empty, then I would say that it probably shouldn't fire recalc but then I'm not surprised if it does

Yes indeed, in my experience, login does regenerate the sales order, as things like price levels might change.

Ok. Thanks!