In the times of the Gemara bathing was a major undertaking, and one could get heavily involved with it and miss the time of praying. As a result of this possibility, Chazal decreed that it was forbidden to enter a bathhouse before Minchah. However, bathing before the designated time for Shacharis was permitted (Shulchan Aruch 89,7).
Today with indoor plumbing, bathing has become faster and more simplified, and we are no longer worried that a person will get so caught up with washing that he will miss the designated time of tefillah. However, since a person should not involve himself with his personal needs before he has prayed, he should try to avoid showering before Shacharis, if possible. If one feels he needs to wash before praying, or if this is the only time he can do so, he may shower before prayer (Responsa Az Nidbaru 6,43).
Some people have the custom to go to the mikvah before praying. Showering beforehand is a courtesy to others who will later enter the mikvah, and also removes any dirt from the body which might invalidate the tevilah. In this case, showering beforehand is considered to be a mitzvah, and someone who goes to the mikvah should certainly shower beforehand (Responsa Shevet Halevei 9,1,2).
Text Copyright © 2013 by Rabbi Daniel Travis and Torah.org