My observation is that it depends on the sail cut on which way you roll it up.
For example: a H16 mainsail rolls best from the bottom up ... a TheMightyHobie18 from the top/head down .... P19MX from the top down.
and yes, I have to unroll both my TheMightyHobie18 and P19MX mainsails before hoisting them up the stick ...
So try rolling your mainsail both ways and verify which direction works best to keep the battens straight and flat, no creases, neatest finished package that fits best into your storage box.
I've seen the PVC trick ... I use it w/ my jib and then place that package inside my mainsail as I roll the main up. It makes a nice neat package in the sailbag, easy to carry, and to get up into my sail storage area in my garage loft
HarryMurphey