My personal feeling is that if a person uses no thumb on the strike ball, no thumb should be used on the spare ball. I know we're talking on hands, but imo a thumb is equatable to a hand. Iunno if Belmo uses his thumb on spares or not, I don't get to watch televised, but I know in leagues where I bowl we enforce Thumb or No Thumb throughout the shot, and it forced guys to learn how to pick up spares with no thumb.
Current in-the-bag-at-random:
Track Robo Rule - OOB - 4.5 inch span
Black Widow - OOB - 4 inch span
Hammer Taboo Pearl - OOB - 4 inch span
Midnight Vibe - OOB - 4.25 inch span
Infinite One - OOB - 4 inch span
Complete NV - OOB - 4.25 inch span
Black Widow Sting - OOB - 4 inch span