Here's my interpretation of transposing. The transpose feature is there to be used, but not abused. When I first started playing, I transposed almost all the time. The only key I could play in was G, and a little in C. And I was told the same thing, don't take advantage of the transpose button. Over last 6 years I've learned every key that we use in church. C, D, Eb, F, G, Ab, and Bb. The remaining keys we have NEVER used and when we learn a new song thats in a key we don't know, we transpose the sheet music in the next available key. Even songs that are written in keys we know we usually end up changing anyway because it's too high or too low. So IMHO, learning a key I'll never use almost seems reduntant because if its never used, I won't remember the things I learned in that key. But for the record, I haven't used the transpose button in probably 5 years.
So my point is, if you're just starting out on the piano, the transpose button will help you along, but don't use it so much that you depend on it. While you're transposing in church services, learn how to play in the keys you want to learn during your practice time. If you want to learn all 12 keys, take 1 month for each key. Say in June you want to learn the key of Db, take that whole month and every day take some time out to sit down and play songs in that key. Then in July find another key you don't know and do the same thing.
I'm not going to tell anyone to learn to play in all keys because I don't know every key myself. But at least learn the keys that you know you'll be playing in the most and go from there.
