Depends, whats the exact query you are using to check the user?
If commonly queried, then absolutely.
What I like to go back through all of my queries I run, pull out the WHERE clause into a text file, 1 per line and have a quick glance at whats commonly used. Try to build as few indexes as possible to suite them all. For example:
users WHERE username = '$user'
users WHERE username = '$user' AND status = '$status'
other_table WHERE startdate = '$start'
other_table WHERE startdate >= '$start' AND enddate <= '$end'
I would make a single index for users with username, status (username first status second because that's the order in the query) and a single index on other_table startdate, enddate for the same reason.
Now if I have a new query come up looking only at enddate, that index really wont satisfy the query because it looks at startdate first.
EDIT: And I found this gem for you: http://www.sitepoint.com/using-explain-to-write-better-mysql-queries/