The argument for showering first thing in the morning, before going to work is that because you will have slept in your own perspiration during the night, and of course shed dead skin, that it is simply not pleasant to walk around all day feeling dirty and smelly. Therefore a shower in the morning, especially if you have a morning exercise routine, will set you up for the day; increasing your confidence in social situations and preparing you for any unforeseen events such as going for a drink with friends after work.
A cold shower, or more specifically turning your shower colder during the last few minutes of a shower has some health benefits: Warm water will send blood rushing to your skin, cold water will send it rushing to your internal organs, a combination of this i.e. Starting with warm and ending with cold will boost the circulation and can help avoid conditions such as hypertension, varicose veins, acne and hardness of the arteries. Therefore the best advice would be to have a normal shower of an evening and another of a morning, with the last few minutes turned to the cold setting.