I would suggest you politely decline the request. Your practice network needs to be as secure as you possibly can make it. Giving out the wireless password is equivalent to running an ethernet cable out through the reception area and connecting it to the drug rep's laptop. They are on the network. It does not matter if they are wired or wireless, it is the same security risk. You have no idea what is on
the rep's laptop, there could be viruses or Trojan horses, unknown to the laptop user.
While considering this question you need to ask yourself why you have a wireless network in the practice? Sometimes Wi-Fi comes with the broadband router and is switched on by default. If all your laptops and desktops are connected to your server by cables, then consider configuring the broadband router to disable Wi-Fi. It is safer not to have wireless. If you can't cable your practice for architectural or structural reasons then ensure your Wi-Fi has the best possible security protocol in place and use a strong password. Get advice on securing your Wi-Fi network from your IT support company.