Screen Sharing; NOT via iChat

Question

I have a G5 non-Intel iMac and a MacBook, both running 10.5.1.
I cannot screen share between these computers unless I click the radio button for Allow all incoming connections under Firewall in the Security Preference Pane, i.e., shutting off my Mac firewall. With the firewall off, all is just hunky-dory.
With the radio button for Allow only essential services pushed and Screen Sharing and Printer Sharing showing in the window below as being activated, however, I cannot screen share.
Under Sharing in Preferences, I have Screen Sharing checked and the radio button set for Allow Access for All Users.
Shouldn't I be able to Screen Share with the Allow only essential services pushed and Screen Sharing activated in the Sharing preference pane?
Can anyone screen share that way or must you totally shut off the firewall each time???
Many thanks,
Bob

Answer 1

Thanks for responding, Topher.
I knew in Tiger how to open/close ports, in the Sharing pane, but that is missing in Leopard. Or have I missed something???
Also, if you use the + button in the firewall, you have to navigate to Screen Sharing in order to add it. I don't think that is possible is it? Where would you navigate to in order to add it. I could find Screen Sharing only as a box to be checked under the Sharing pane in Prefs.
Many thanks for the help,
Bob