Hi, and welcome to the JRA!

The way I see it, the mast *can* be tapered as it needs less bending strength nearer the top.  But it doesn't *have* to be tapered.  Reasons to have taper include:
1.  It looks better!
2.  Less windage high up
3.  (depending on construction) less weight high up

Reasons not to have taper include:
1.  It can add complexity to the construction (depending on method)
2.  The reasons you describe, to do with advantage of consistent diameter

One popular design is to have a cyclindrical lower section, and a tapered top section.  This can yield all the advantages of taper, and means that any problems with varying diameter are limited to the top part of the sail when carring most of the canvas.

Really, it comes down to your chosen mast material and construction.  There's certainly nothing to stop you from keeping it cylindrical all the way to the top.

Good luck with the conversion - keep us posted!