If you're planning to race it, tapered sheets are a no-no:
16.2 Sheets and lines:
a) Any diameter and quality of sheets and lines
may be used unless prescribed otherwise in
these rules or in the individual boat class rules.
b) Any sheet or lines made in whole or part of
wire are prohibited, except as supplied by the
HOBIE CAT CO. Sheets and lines, with the exception
of the spinnaker halyard systems, are
not permitted to be tapered unless allowed in
the individual boat Class Rules.
There's no real reason to taper the main. In fact, the smaller line doesn't hold as well in the cleat and you wouldn't be able to flip the sheet end-for-end when it starts to wear at the cleat.
I use a 37' main sheet - same as my jib sheet.