Firewall Blocking Printer Sharing

I have a laser printer directly attached to an iMac via USB, which I have added to the printers & fax preference pane and selected for sharing. This works fine for local printing.
However, when I try to print to it from another machine on the local network (also OS X10.5.1), whilst I can add the shared printer to the print & fax preference pane on that machine, I am unable to print from it.
I was suspicious that this may be an issue related to the firewall so began to look into this aspect.
I had the 'Allow only essential services' option selected under the firewall option on the printer sharing machine and 'Printer Sharing' was entered (automatically) in the application box below it. This is added by OS X directly when you tick the 'share this printer' box in the printer & fax preference pane, if you unselect it then it will automatically remove this entry from the firewall pane.
I then checked the appfirewall.log and found the application firewall was in fact blocking the incoming print request from the remote machine... <Firewall46: Deny cupsd connecting from xxx.xxx.xxx.xxx:49217 uid = 0 proto=6> - which is the machine I was trying to print from.
So even though it was set to allow printer sharing through the firewall it was being blocked by the firewall.
When I then set the firewall option to 'Allow all incoming connections' the print quickly showed up in the local machine print queue and promptly printed!
I do not want to set my firewall to this setting long-term just so that printing will work.
So my question is, how do I configure the firewall to allow shared printing?
TIA, Simon