It seems to me 5-finger exercises are usually presented in the key C major, but I thought it might be good to practice them in all keys. Bb and Eb major are difficult for me, because my third fingers get stuck between the black keys when hitting the white keys. If I angle my forearms a bit it seems to help, because my fingertips have a bit more room to fit between the keys. In Bb the problem is finger 5 L.H. and finger 1 R.H. have trouble reaching Bb. Forearm and wrist adjustments seem to help. Hoewever, both of these keys feel awkward. Can anyone offer any other advice? Does anyone recommend to not do 5-finger exercises in all keys?
You certainly should practise 5-finger exercises in all keys.
If your fingers are getting tangled in the black keys, practice slower, try bending your fingers more, and try moving your hand (and arm) forwards and backwards when your thumb needs to reach a black key.
All this will feel awkward, in the sense that if you wanted to play the notes in a piece of music you would normally use the same fingering as a Bb or Eb major scale. But one day, that won't be a practical option and you will need to play a 4-note chord like Eb - Gb - A natural - Eb without getting tied in knots - and maybe add a C natural as well, or a 5-note dominant 7th chord like D# - F# - A - B - D#.
Unless your fingers are very wide, and the middle one is actually squeezing betweed Db and Eb, you should be able to play using all five. Thumb will start on Bb (for the Bb exercise), and your hand ought to be in a straight line with your forearm. Moving so that the angle from above changes probably won't help much. In fact, normally, you should be able to place all five digits on all five keys, and lift each separately.
All the above presumes that your fingers will fit without getting jammed - something I've never seen! If that's the case, you'll have to press the white keys just where the black keys ends are. Of course, your fingers are nicely bent, so the tips press them down. Aren't they?!
To prepare for full scales, though, it's a good idea to start with middle finger on Bb, then thumb under onto C. Thus, it's only a four finger exercise, but thumb on a black key is often something to avoid. Not always possible, obviously, but given the option to play a black key with anything but thumb - take it.