On the specific point of "if exists", the reason to use "*" in that case is so SQL server can pick which index it wants to use, instead of putting in a specific column name and thus possibly forcing it to use a specific index. It's a minor speed advantage.

Using "select 1" means SQL Server can use whatever index it wants, and doesn't have to look up column names. Again, a very minor improvement, but every bit counts in some busy servers.