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whilst I believe that composers of music should write what they are moved to write, you might consider sitting down with the instrumentalists (and not just the tuba player) you are writing for and see (or rather hear) what is practically possible for them to play. It does make a difference, if you are writing for young players or learners you need to consider issues such as range in regard to the ability of the players and the instruments they have available to them.
for a more advanced player, it is fair to write a passage and ask can you play it? I am sure most would take up such a challenge to push and develop their technique

bloke wrote:There is a passage in Prokofiev's Romeo and Juliet (Montagues and Capulets) that simply consists of a minor scale in eighth notes starting down in the very low range, ascending into the middle range, and then descending back down into the very low range. The speed of the quarter note beats is only about 100 - 110 bpm...

...Tuba players have to practice that passage quite a bit in order to execute it with any sort of good solid sound (musicality aside).

That's why it appears on many audition rep lists. The low note is F 4 ledger lines below the staff. Very reasonable for someone who wishes to support an orchestra with their sound.