I grow violas every year, and if left they will still produce the odd flower after season. I never deadhead mine, some of the seed pods burst and new plants spring up here and there but I mostly collect the seeds, dry them and replant them when its the right time of year. I just did all my summer violas yesterday. I can keep the same plants going this way year upon year, and I can single out the odd hybrid/cross pollenation in the process.
They really are one of the most hardy plants ive ever grown.