Do not use them in secondary because it might add too much sugar.
It depends on what your end goal is. Fermented and non-fermented cherries will impart difference flavors.
If you are looking to make a "fun wine", like a Cherry-Merlot, go with post-fermentation. Start with clear and CO2 free wine. Stabilize the wine and add the cherries. Once a week, stir the wine to distribute the cherry flavor, and taste. Rack the wine off the cherries when you're satisfied. However, grape solids start to decompose post-fermentation, so I'd expect the cherries to do so as well, so I'd not leave the cherries in for more than 2 to 3 weeks. If the cherry flavor isn't strong enough, you can either add a different batch of cherries or add cherry juice, although the juice may have a lot of natural sugar.
My comments are not very precise, but we're dealing with a lot of unknowns, so you'll have to wing it to some extent. Other folk may have other methods to try.
OTOH, If you want a dry red with strong cherry notes, add during fermentation.