Yes it is safe to reset phones that came with 10.
You can still run the WDRT and the OTC updater also.
Phones that are running 8.1 and below should not be reset unless they can be updated to 10 using the OTC updater. Lumia 1520, 930, 640, 640XL, 730, 735, 830, 532, 535, 540, 635 1GB, 636 1GB, 638 1GB, 430, 435, BLU Win HD w510u, BLU Win HD LTE x150q, MCJ Madosma Q501
When 8.1 phones that can not be updated are reset the phone will not accept a Microsoft account when it is restarted.. end result is a brick