It all depends on your agreement, and if this is a competitive situation.
In several of my short sale listings that had competitive offers, one of the things that set the buyers apart is their willingness to open escrow upon acceptance of the offer by the seller, and pending approval by the short sale bank. The reason for this is that it show how serious the buyers are, how willing they are to commit to the waiting period.
It is not unusual for buyers of short sales to back out of the agreement if they don't want to wait much longer than 45 days to receive short sale approval. When they do that, the short sale process starts all over again, which is frustrating for both seller and the listing agent.
There are no guarantees as to how long it will take to receive short sale approval. The listing agent and seller will likely want to have a longer waiting period beyond the standard 45 days on the short sale purchase addendum. Furthermore, as show of good faith, they would want the buyer to open escrow upon acceptance of offer. Then both buyer and seller wait together.
If the buyer isn't willing to wait, or to open escrow, especially in a competitive situation, then the buyer is writing a weak offer from the seller's and the listing agent's perspective.