I dont see any huge problem with that, but....
How big a potential problem it is, has more to do with the nute balance than with any bio hazard between changes.
If you are feeding at the correct EC/PPM level, and the PH is staying in range between changes, then it would be pretty much a zero issue.
On the other hand, if you are feeding at too hi or low an EC/PPM, and/or the PH goes out of range, then the nutes can get seriously out of balance between rez changes. That means that 10% left over will be low on some nutrients, and hi on others. That imbalance can add up over several rez changes.
Still, 10% hold over probably wont get serious unless you're really screwing other things up
If it bothers you, you can always do an extra rinse when you change the rez. Drain as usual, then refill with plain water and drain again right away. Then top off as normal with your mixed, PH'ed nutes. That will get you from 10% down to 1% or so.
Do you monitor EC/PPM and PH between rez changes? If not, then you really should. Its the only way to be sure you're feeding at the correct levels.