It's a policy issue. I would not allow it. As noted, if they buy their checks on the Internet or from an ad in the Sunday paper, there is little or nothing you can do about it. However, if they buy them from your bank you may be complicit if someone reasonably believes the "agent" actually owns the account.
Again, a matter of opinion and policy, but I would allow an attorney-in-fact's name to be put on a check if the fiduciary capacity was indicated next to the name. I hold my Mom's POA and have signed checks carrying only her name for years without being questioned. However, one retail store told me "Your ID is no good unless your name is on the check." Since Mom needed the TV I did not want to begin a debate, but simply pointed to the language above the signature line: my name, attorney-in-fact. They thought that was peachy.
In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.