I think this highlights the difficulties of doing low resolution spectroscopy at ground level. I did see your post on the ARAS site David but could find no references to your question via various Google searches. It seems planes are not an issue for the professionals.
I think in addition to the issues covered above if you want to get an accurate continuum you need to operate with a wide slit. That is one where the slit is significantly greater that the seeing disk so as to admit 99.99% + of the incident star light. Thus your spectral resolution will be seeing/guiding limited.
Obviously, this will reduce your spectral resolution compared to a narrow slit where it defines the effective resolution (all else being equal). So you trade off photometric accuracy for resolution
I think your science goals will dictate which is needed and if it is both then two set of observation may well be needed.
Regards Andrew
PS see here http://www.astrosurf.com/buil/alpy600/photometric_slit.htm