In my experience, you always take a cashier's check for 10% of the first overbid amount. I'm sure there are exceptions, but with a cashier's check there are no exceptions.
The original bidder did not show up. The judge said OK Mr. New Bidder, we'll take your offer but we need a cashier's check. Go to the nearest bank to get one. Up jumped the listing agent. Your honor, while he is getting the check, can we call the original buyer to see if he wants to bid up? Judge said OK. (Huh? Really?)
The new bidder went to a bank and was delayed a little in getting the check. Finally he got the cashier's check. What happened next? He went back to court only to hear that they were trying to get a hold of the original buyer. (Huh? What?) Lo and behold, the original buyer does show up! (What?) Now there was competition and the original buyer beats him.
What's the lesson here? Take a cashier's check...always. Don't take it easy. Don't listen to the listing agent in this regard. What's he going to say when he screws up? "Sorry." And you and your client lose.