I bought a Korg auto-chromatic tuner about for the sole purpose of tuning my autoharp. It's a beauty. It cost me 140 dollars fifteen years ago. I have since been using it on my guitars as well when needed, as when it's noisy. I can tune the guitar by ear just fine at home (when it's quiet). But I don't think my hearing is quite what it was when I was young. It is a mild difference which many people experience to some extent as they age. IMHO, rather than a crutch which will destroy your tuning ability, it is a tool to enhance it, and may become more useful as I age.
I recently bought a cheap, (30 bucks), digital chromatic tuner, so I could have a tuner in both my guitar cases. I won't mention the brand. I don't like the display, (as others mention), and it is nowhere near as sensitive as the old Korg. Even in a quiet room, with it plugged into the pickup it takes twice as long for me to get in tune. You get what you pay for.

McGrath of Harlow- I agree completely about the misnomer of "quiet folk music". I recently spent some time trying to follow Bill Sables on the melodion. I had a hard enough time trying to figure out where the key changes were before he went back to the original key! I don't think I could have tuned with harmonics.

For myself,sometimes I have to decide whether I want to play in tune, or "just play regular". BlueJay