Unless you have something very unique which is highly sought after, guitars make lousy investments. If a guitar appreciates, it's usually due solely to inflation. And Taylor isn't old enough to have attained the kind of historic value of Martin or Gibson.
Of course, modifications are a personal matter, and if you don't feel comfortable adding a pickup to your Taylor, then it's probably better to leave it alone or use a pickup which can be removed without leaving tell-tale signs. FWIW, though, I have an '83 615 which has a pickup with a strap jack, and I couldn't care less whether it's lost any value. I've had that guitar for 25 years, and it's not going anywhere.