I would second the recommendation for trying Enfuse - in fact I would suggest you try the whole process in Hugin, which incorporates Enfuse. Hugin is free, open-source software available on most platforms. It gives much more control in aligning and transforming images than Photoshop's Photomerge function (which is not an option for you anyway I don't think in the original Creative Suite).
The algorithms in Enfuse are very good at avoiding any halos, which are easy to create when manually masking between layers with a brush in PS.
As far as special considerations doing this with scanned LF film: I think it depends on how you mount the film - I've found with my Epson V700 that if I scan a 6x7 negative multiple times, each scan will look slightly distorted compared to the others, when using the OEM Epson holder. I believe the heat from the lamp makes the negative move a little over time. If you were dry-mounting or wet-mounting onto a piece of glass, I think this would be much less of an issue.
I have read comments about the motor that drives the ccd in Epson scanners not having perfect registration or identical movement speed from scan-to-scan, but I can't comment on this from personal experience. I would point out, however, that Lasersoft must believe it is accurate enough since they offer a feature in Silverfast that combines multiple scans of one negative...