Does installing packages manually with dpkg prevent a future upgrade path?

No. Installing packages from the official repositories using dpkg will not give you any problems down the road.

In fact, from a technical point of view, installing a package via dpkg is no different than if you had run sudo apt-get install package. The same things happen.

The only problem with installing packages with dpkg is that you will have to make sure that the dependencies are installed in the right order. If not, you could wind up with a corrupted package database.

I know this happens from personal experience...

Just for completeness: when you upgrade, if your packages are not supported anymore, or incompatible with newer packages (broken dependencies, etc), then the upgrade process will ask you if you want to remove those packages. I'm not sure what happens if you don't uninstall them - probably they will not work anymore.

In any case, you will be able to upgrade.