I've stayed onsite a lot, and sometimes they will put the hold for incidentals on the card, and other times they don't. The amount varies too, as it is not a flat figure -- it's based on a percentage of the room total, or one night's total.
If you are all paid up in advance, before arriving, you have the option to not have the incidentals hold placed on your card if you don't want to leave a card on file. There have been times when an incidentals hold was not supposed to be placed on my card because I was paid up in advance, and it was placed on the card anyway -- much to my surprise when I returned home and saw the card transaction alert! Other times there was no hold because I was paid up and I told them I did not want to leave a card on file.
Sometimes the booking system will automatically apply the hold on your card when you check in, and I think the Cast Member has to enter in a code to reverse it or cancel it out before that happens. With certain types of cards used to pay for the room, the hold might be automatic and require the CM to cancel it out, and with other types of cards used to pay for the room ( a Disney Visa, for example), the hold might not be placed at all if you're already paid up.
So, I guess the bottom line is, if the incidentals hold is a concern to you (because it can be lofty), ask the CM when you check in to tell you exactly what is happening and if you are going to be charged. Even if they tell you that there will be no hold because you're paid up and don't wish to leave a card on file, keep it in the back of your mind that there may be a hold placed anyway, by mistake, and check your card's transactions often -- during and after your trip!