If you’re installing TFS2008 using SQL Server 2008, you MUST slipstream your installation. However, if you’re installing on Windows Server 2008, and using SQL Server 2005, you do NOT need to slipstream.
If you’re installing WSS 3.0 on the server during the installation process (such as for a clean install), then you really ought to to create a slipstream version of the TFS install with TFS SP1. Reason? Without the slipstream, you have to install WSS 3.0 with Sp2 on your own, and configure it. And it takes less time to create the slipstream than to install WSS 3.0 with Sp2.
So, create a slipstream and install it! You can find slipstreaming instructions all over the web, but here’s a good one from my friend (and TFS super-genius) Martin Woodward. You can access it at: creating a TFS 2008 SP1 slipstreamed install.