We have a few databases that are stuck in legacy mode at SQL Server 2000. I commonly use SQL Server Management Studio from SQL Server 2005 to query the data in these dbs from my desktop, but have been logging in to the server to use the SQL Server 2000 tools installed there to manage permissions and perform other admin tasks.
Would it be a problem to use SQL Server 2005 from my client machine for this purpose?
No, you should be good. There are a couple of things you might not be able to do (such as playing with DTS) but by and large you're good. I did the same thing until we upgraded our servers to 2005. Now am doing the same thing using SSMS 2008 with SQL 2005 servers.
In my experience, if the operation you are trying to do can't or should not be done with SQMS, it will not let you do it. You will get an exception. If it lets you perform the operation, it should be fine.
You may also want to install the Microsoft SQL Server 2005 Backward Compatibility Components.