As long as it does not obscure the card and is not detectable from the back or when thumbing through the deck while face down, it should be fine.
One issue they need to be careful of is if they are using an ink pen instead of a sharpie, they need to be careful not to press hard enough to dot the back side of the sleeve.
WoW RK2
VS RK2
YGO RK2
PM LV2
Specialist LV2 (formerly demo team)
TO LV1