We used to recommend to customers running the old version of Access Reporter (written in Visual Basic 6) to use a separate box. With the new version of Access Reporter (written in .NET) we find that only customers with very heavy usage (more than 60+ reports per minute) seem to need a separate box.
This version uses .NET remoting is very fast compared with the Visual Basic 6 version (that version polled a database table). The majority of our customers tell us that Access Reporter is running quite happily on the same box as IIS, and SQL Server.