When Does It Make Sense to Do Loss Harvesting?

When does it make sense to do loss harvesting? When is it a prudent and practical strategy? Let's talk about some of the times when it does not make sense. If you don't have any losses, if you've been dollar cost averaging for the past couple of years on a low cost US based index fund odds are even though this year the market is down, your holding is probably still up. So you have to actually have losses in order to be able to loss harvest.