In the last months I came across several customers that were in the process to evaluate, or to deploy Office 365. It usually started with a Office 365 trial, that some of the IT guys started to play around with. Weeks or months later, during the proof-of-concept or during the final deployment, the customer had to choose a Office 365 tenant name. That is the part before .onmicrosoft.com.
I had it multiple times, that the desired tenant name was already taken. Bummer. But the customer wants to move on, so the customer decided to take another another name. For example, they added the post code to the name, or a random string. To their surprise, I put my veto on it. They immediately understood why, after I explained the importance of the tenant name.