Actually, checking for FIXEDPITCH doesn't even help. For some reason this also excludes StdFont() and a load of other fonts that I want (since they're the only ones that don't look awful).
Any ideas? I'm going to investigate further and see if I can find the root cause of this.
FIXEDPITCH actually means MONOSPACE.
I hope terminlogy is correct, this external source (first one googled seems to agree: