You should hopefully be talking to each other at least every couple of days - the real advantage of having visibility of each other's branches comes when you use them to share work on a much smaller timescale.
As for caring about your reviewers' time and sanity, rewriting commits that they may already have seen is the opposite of that IMO. Any decent review tool will let you review a single combined diff for the whole branch, and that's what a reviewer who hasn't been following your progress will use. Meanwhile if a reviewer did happen look at your branch yesterday, taking away their ability to view just the changes since then is doing them no changes. (This is especially true when it comes to applying changes from review feedback - if I requested a couple of small fixes then I want to review a commit where you made those small fixes, I don't want to have to re-review the whole PR because you rebased)
As for caring about your reviewers' time and sanity, rewriting commits that they may already have seen is the opposite of that IMO. Any decent review tool will let you review a single combined diff for the whole branch, and that's what a reviewer who hasn't been following your progress will use. Meanwhile if a reviewer did happen look at your branch yesterday, taking away their ability to view just the changes since then is doing them no changes. (This is especially true when it comes to applying changes from review feedback - if I requested a couple of small fixes then I want to review a commit where you made those small fixes, I don't want to have to re-review the whole PR because you rebased)