Thanks Ken - so if I have the option to enable it, I should?
Readers Digest answer: To be honest, I don't know.
I have only a marginal understanding of the GPS message protocols (NMEA and Garmin's proprietary version). That understanding comes more from the fact that I'm reasonably conversant in a variety of computer and telecommunications protocols from my former life as an IT and telecom professional than it is from any working knowledge of NMEA. That simply means that it's easy for me to get a handle on the basics of any protocol's message set or stack. But it completely begs the question of what each individual packet or message contains and what the receiving program uses the message data for, or even if it uses that data at all.
So, I don't know what nav program you are using. I don't know whether it will use the VTG data even if it's there. And, if it uses that data, I don't know what features it would use that data for or if it can provide those same features by using data from other messages.
Some nav programs may depend on getting the VTG message in order to compute and display your speedometer, although there are other ways to do that without it.
There are two ways to find out if it matters. Turn it on in the receiver, turn on tracking in your nav software and go for a drive. Observe the behaviour of your speedometer and any consistent difference relative to your car's speedometer. When you're done, check the track that was created.
Next, turn it off and do exactly what you did before. Observe to see if the nav program's speedometer changes its behaviour in any way. When you're done, compare the tracks of the two trips.
Or you could just email the tech support folks for your nav software and ask.