No risk at all. A login prompt is only for preventing physical access to the computer. This protection role has been replaced by the FDE and the accompanying PBA (which is much more secure), so any attacker that would break or somehow bypass the FDE could bypass your OS login too.
However, running as a limited user (with autologin on) and having a password-protected administrative account (with autologin off) can be useful, since the account then protects against online-based attacks (virus infections, malicious scripts etc) while the FDE protects against physical (offline-based) attacks.
But remember that anything that your currently logged on account has access to (regardless of password protection), can be accessed by a virus or malicious tool too, so a good idea here could be to also put sensitive data in separate encrypted containers with different passwords. So if you get a infection or a malicious script, then they only get access to whatever you are working with at the moment.
This would mean those containers would be encrypted 2 times on the disk platter, first by FDE and second by the container encryption.