I agree 100%! I have the Garmin 182C and I had installed the unit but left the antenna sitting on the dash under my pilothouse roof (which is 1.75" thick...glass/coring/glass) for two months to make sure the unit was mounted where I wanted before drilling more holes and running more wires. My GPS accuracy was 7-9'. To this day I still check once in a while but after mounting the antenna on the roof the GPS accuracy is still 7-9'.

I also just noticed the newer models are much better for autos. I had an old Lowrance unit in the truck that if mounted on the dash under the windshield was fine but under the roof it would not acquire the birds. The new Garmin works in the house, under the roof of the truck...so I would definitely go without the antenna for most inshore applications. Plus I think with most integrated models if you really had to they have a port to plug in an external antenna if the reception was not adequate.