I have a large stand of Viola striata. Around the middle of October, I run the lawn mower over the entire patch. It seems to benefit the plants and in the Spring, they come back with good foliage and grow vigorously. We live in Baltimore, MD. Our first frost is usually mid-October.
Here is a link to the photo of about 1/3 of my Viola striata planting.

Here is a link that might be useful: Viola striata in the Spring.

The best course of action depends on what species you have. Some are naturally tall, like V. canadensis, so cutting down would be pointless. In general, deadhead the spent blossoms anyway. Only then consider cutting the foliage back. Chances are it will not make next year's flowers any shorter, though. It may, however, be an effective way of solving severe infestations of foliar pests such as red spider mite. Also, as Jill Ann says, removing the foliage seldom does much real harm, so long as the plants are in adequate soil and so long as the crown or rhizome is not destroyed. Man might do that with a lawn mower, shears or secateurs; but nature does it also - with browsers' jaws and teeth.