I've read here that an RSA signature error is irrelevant, but for me, 2004 Jetta, it did matter.
I got locked out of the ECU twice due to this. Had to boot mode flash the eeprom to get it to work again. I then started using a different checksum tool which fixed that error, and have not had an issue since.
Would be nice to have a definitive answer on this, as I use ECUFix..