Hi David,
I guess your choice of technique might depend on why you want to reduce your resolution in this way. Is it to reduce the number of voxels for computational efficiency? If so, I'd try 3dresample, using the -dxyz option. Keep the in-plane voxel dimensions the same, and increase your slice thickness five-fold. One thing to watch out for in 3dresample is that sometimes activation shifts to one side slightly after resampling. To prevent this, you may need to use 3dZeropad to add slices of zeros to the edges before resampling.
On the other hand, if your intent is to smooth out the data, you might want to try one of the 3dmerge smoothing options. Hope this helps.
Sally