I never give out blank checks. We do virtually all reimbursement, but there are times when we pay by check up front (ex: to pay for an assembly or mail order purchase for the school). If we had a particularly large purchase that the volunteer didn't want to front, or that we couldn't prewrite the check for, I would write a check for the approximate amount to the vendor. Then the volunteer would only need to front the (small) difference. This also limits the PTO's exposure and risk to only the amount the check is written for (as opposed to a completely blank check).
Only let key people purchase things. If they get reimbursed in a very timely fashion, most people don't seem to mind paying up front. For really large amounts, I have been know to occasionally give a blank check, but I know in advance the Who, What, Where and Why's. Find a few people you can trust and they are to be the only ones that can make purchases.
Do you ever give a check made out to a Committe chair or someone who has volunteered to make purchases for a particular event before the event? We do it strictly by reimbursement, with reciepts, but we are trying to get new people involved in taking over some of our events and I don't know how people feel about using their own money up front. We've had theft problems in the past and I'd hate to leave us open for that again, on the other hand we need new blood. Any advise? Thanks