is it considered bad form? Plenty of modern masters do it and i think i'm gonna start, transposing instruments are a complete thorn in my side, they slow down my workrate and have often disrupted my train of thought- f**k the haters, c is in, needless transposition is out!
apparently stravinsky never composed away from the piano and i think it's safe to say it worked for him, but for more modern work i think it's important to be able work with your mind's ear.