Since 32-bit applications run mostly fine on 64-bit Windows are there any scenarious where I will have 32-bit Microsoft Search IFilter consumers on a 64-bit machine? Can I just install the 64-bit version or do I need both?
I would say that you only need the 64bit version on the 64bit platform.
Looking at it from another perspective, why would you run 32bit SharePoint on a 64bit OS? (Other than some strange compatibility issue) SharePoint loves memory, the more the merrier, and 64bit lays the ground for that.