OK, now that I've had some sleep, time for some calculations.
Comparing my 3.77b (which didn't wait) to my 3.75b (which did), waiting is worth about 3300 PIV on Normal, and loses about 2m from the spell bonus as Reimu. It does not make a difference on the next border's timing (if it did, I'd wait regardless).
There are 3 blue border bonuses after (bonus of PIV*40, since all border bonuses add twice what they display), for a gain of 3300*40*3 = about 400k.
And that's before accounting for the value of point items (and their border multiplier-- in my run, 4.4 for the first spell's items, 4.5 for the second spell's items, 4.6 for the 3rd/4th/final spells' items). If I calculated correctly, Tensei drops 30 items after her first 4 spells, and 60 after her final (none after the survival). In that case, 3300 PIV is worth about 3300*4.6*150, or 2.3 million.
In other words, that small delay is worth a net gain of a few 100k on Normal. It should be worth significantly more on Hard because of a larger cancel, a higher border multiplier (5.3-5.8 in the Hard 7b, I think), and another border and 3 point item sets during the Last Spells.
Edit:
Meanwhile, on her 2nd spell (still Normal), canceling during the blue lasers seems to be worth about 2500 PIV, while canceling during the pink lasers seems to be worth about 6000 PIV, and during yellow lasers around 7000-10000 (!) PIV (plus 2000+ from the extra graze) for again, a loss of only ~2m in the spell bonus. Definitely worth waiting on that one too. The 10k cancel is hard to time, though-- I normally get around 8000 from the cancel (still worth waiting over getting 6000).
...actually, stalling the 2nd spell until the final yellow wave is worth about 8000 more PIV from grazing, for a loss of ~10m spell bonus. With 2 borders and 120 items for the PIV to affect, it seems too close to call on which is better.
Edit 3:
Actually, blue borders give PIV*40 (not PIV*400) points. This changes the calculation. The Tensei cancels should still be worth losing ~2m each from spell bonuses, but only slightly worth it in the case of her 1st.