Should I replace both wheels/rims if I convert the front to a dynamo hub? The title mostly says it all. Is there a reason to or a reason not to replace the back wheel or rim if I replace the front wheel to get a dynamo hub?

The only reason would be aesthetics, to keep the wheels the same, but that doesn't sound like a good enough reason for me.

My bike is a hybrid Specialized that is maybe 3 years old, but with only 100 or so miles on it. I have been commuting to work and looking into getting dynamo powered lights.
You should replace the rear wheel only if there is another well justified reason to do so, but generally both wheels are mostly independent, so you can perfectly change only the front.

The first scenario I think of is where you are changing only the front hub (and spokes) and keeping the front rim. Why would you change the rear one?

Easier task is to replace the front wheel (i.e. rim+hub+spokes), sometimes they are even sold this way, so it's only matter of mounting the tire and installing the cables.

Rear wheel can be changed at this time if there are reasons on its own, for example the hub or rim are already close to the end of the useful life or are worn out completely.

Other reasons may be: 


You bought a wheelset that included both, or your current rear hub is a dynamo (and you don´t want two dynamos in one bike).
The rim you are putting in front is a different width or size, or is intended for a different use (e.g. from super light rims to utility ones) and you want to match the rear for usage.