Hi gros21,
I would think SonarTRX would be used mainly for visualization of side scan sonar recordings and it's ability to combine multiple recordings into a single "bottom structure map". Humminbird PC has had the ability to View in Goggle Earth for many years (although there were periods where it was dependent on using older versions of Goggle Earth). Viewing GPS data in Goggle Earth allows visualization of waypoints, routes and tracks. Seeing bottom maps or waypoints overlaid on a larger map helps to put them in perspective and gives them more meaning.
There are many different programs that allow you to edit GPS data. I still use Garmin Mapsource. Which ever editing program you use is up to you. Just make sure you can get your edited GPS data back onto your Humminbird unit. In most cases that would mean getting your edited GPS data saved into a GPX file format. That would let you open the file in Humminbird PC so you could transfer it to your unit. Also you should verify your edited GPS data by making sure that something like a waypoint for your local dock still shows up where it is should be when you go back out the next time.
Darrell
Edit - Here is a link on getting a Google Earth file converted to a GPX file (which has a link to GPSBabel) ...
http://support.google.com/earth/bin/answer.py?hl=en&answer=182525