Late Winter/Early Spring before the new growth starts. Do you know the species? L. spicata spreads by rhizome very aggressively; L. muscari is clumping and non-aggressive. One way to tell the difference is usually the flower of L. muscari is medium to dark purple whereas L. spicata's flowers are lighter purple (lavender) to white. Also, L. spicata has thinner leaves and looks more grassy than L. muscari.

If you have L. spicata it will probably surround the roses and might interfere with their root systems -- so would be best to get rid of it -- though that is easier said than done! L. spicata can be a curse. Good luck!

I usually leave mine standing over winter, but I don't know if it's necessary. I have L. muscari, which stays relatively green for me over winter (though a snow dump wrecks the foliage, of course...). One of DH's chores in the spring is cutting down all the ornamental grasses, so I just leave the liriope for him to cut down while he's working on the grasses.

If you cut it down in Fall, you take away its "evergreen" attractiveness, but otherwise you won't harm it (unless you are in a zone in which it is marginally hardy, then you may help it survive by leaving the foliage as winter protection). It comes down to your personal preference; if it starts looking hideous during the winter, it may be better to get rid of the ugly. A lot of times people wait too long into Spring and cut the new growth which then makes for a hideous looking plant all season long.