As stated above, if you only use the 50 to start out with, and really learn what that lens can do for you, you will better be able to use any other lens you acquire. I did that when starting out: I had only my normal lens for the first few years and really wrung it out before getting more. And if I could only have one lens to use all the time the normal lens would be it.
+1

I learned on a screwmount Pentax with a 55mm, f2. I love the older screwmount lenses I have (all Pentax). I haven't used any of the ones between those and the most recent lenses from them, however.
Can't count how many times my dad said to zoom with my feet when I asked about a zoom lens. Really does help you see possibilities, though.