I have a Steinway baby grand which I practice on at home but there are times I cannot use it because of other people in the house. I only play classical music. I'm thinking of buying a touch sensitive keyboard to use with headphones for those times for the sole purpose of fingering and learning new music. Does anybody use a digital piano to learn classical music but once it's "in the hands" transfer it to an acoustic? Does anybody see a downside to this?
That is a great idea for extending your practice opportunities without disturbing others in the home!
In addition to touch sensitivity - I would recommend that you get a fully weighted hammer action keyboard like the ones more commonly found on digital pianos (less common on keyboard controllers and synthesisers). Most all of these will also be velocity sensitive as well - meaning the harder you press, the louder the sound, although that may not be as important to you just for practicing fingering (but might for learning new music).
The various manufactures use different mechanisms to attempt to emulate the feel of an acoustic piano in a manner that renders a similar playing experience along with the ability to express the music in a similar manner as on an acoustic piano. Varying versions of these digital pianos have varying degrees of success with achieving the feel of an acoustic piano - so you should try them out before you choose. Try to find one that feels as much like your Baby Grand as possible.
I would be afraid that without the graded, weighted hammer action feel, you might not benefit as much from practicing on such a keyboard or worse, may develop some adaptive sloppiness in your technique that might not transfer well back to your Steinway.
I have a relatively inexpensive Yamaha 88 key digital piano with a headphone jack that feels very realistic. It's a very different feel from my Casio keyboard with touch sensitivity but lacking the graded hammer action or weighted keys.
Good luck and have fun learning new music late at night or the wee hours of the morning.