I don't think I explained that clearly. I didn't mean phone conversations would cut off after an hour. When your phone registers with a VoIP server, the router keeps the "NAT hole" open for typically the length of the Assured UDP timeout. This way, the VoIP server may deliver incoming calls to you. As long as the phone's registration interval is less than the Assured UDP Timeout, everything will work properly.
With SIP Helper turned on, the router will not respect the Assured UDP Timeout and instead uses 3600 seconds. This means that your phone has one hour to re-register and will possibly make Linksys devices workish with their default settings.
The reason this is not optimal is that if for example your ISP changes your public IP address, it could potentially take an hour before you can receive incoming calls, or worse, it could require human intervention to reboot things if you're affected by the bug I mentioned before.
Obviously if things work well for you then there's no need to tinker with anything. My point was that my opinion is the same as the general opinion in this thread - leave my SIP alone!
m.