This article is from the Mac Programming FAQ, by Jon Watte firstname.lastname@example.org with numerous contributions by others.
It would be foolish to lock yourself out of the many benefits
the System 7 API provides for software that you start to write
now. Some of the System 6 and older things (likely SFGetFile and
wdRefNums among others) will be phased out of the interfaces and
lose support; especially on future platforms.
The installed base of System 7 is larger than that of System 6;
this is not surprising because Apple has been shipping System 7
for several years with all new machines, including the LCII,
Classic II, Performas and Color Classic. Another argument is
that newer computer owners (having System 7) are much more
likely to buy new software than old computer owners who have
systems that already do what they want them to.
The added work to support both System 6 and System 7 is
significant; if you have the time and money you may want to do
it, but only supporting System 6 and not System 7 is doomed to
fail in the market of today.
Some may call this position subjective; I call it business sense
based on market demographics. A rule of thumb may be that if you
target color machines only, you can just as well demand System 7