Hi Rocco,
I believe for both GE and Philips MRI scanners, it depends on whether you've selected sequential or interleaved when you acquired the data.  If you're doing interleaved acquisitions, you can use 'alt-asc'. If you have selected sequential, then use 'seq-asc' (it might be possible to do descending sequential, but I don't know).

For interleaved, Siemens is a bit weird in that interleaved acquisitions start on the first slice if you have an odd number of total slices, but on the second slice if you have an even number.  There is also an extra 20msec gap on Siemens scanners that have PACE installed, and the option 'siemens-alt-asc' accounts for this.  Its a tiny tiny effect and you'd probably get the same result either way.
Erik