Nope, there is no problem with doing that. In fact, I just added another Windows XP computer to my shared printer that is on a Windows 7 computer a few days ago.
Just add the computers to the same workgroup and make sure the printer is set to be shared. Then, in Windows 7, open "Computer" and click the network button on the left and select the Windows XP computer that has the printer. Connect to the printer and then remember to go to Control Panel->Printers and set it as the default printer.