When I get a new client, I talk to them to find out whether they already have accounts set up, whether they want me to set up the accounts for them, or whether they want to set them up themselves.
With the folks that I've worked with, most have just had me set everything up for them, including their shipping, billing and tax information. I have had 1 or 2 clients that have had me create the accounts for them and then I just sent them directions for how to fill in their tax information, and I just supplied the username and password.
If I'm setting up a lot of accounts, I'll usually create a separate gmail account for them, and then when everything is done I'll give them a spreadsheet with all the usernames, passwords and login URLs to all their accounts (including the email) and will set up the gmail account to forward emails to their actual email address. That way they get something that's ready to go rather than having to create the accounts themselves or confirm email addresses for me before I can continue.
That said, if they have an account or want to do it all themselves, you can easily just hand over the MOBI file for them. I recommend just doing whatever your client is most comfortable with, and the easiest way to find out is to ask. It can potentially provide you with additional revenue sources; you could charge one amount to do the conversion and another amount to set up their accounts (I wouldn't differentiate between setting up an account w/all their info or creating their account and having them enter tax information, though…)