I don't know exactly how long gives you too much dispersion (obviously depends on the fiber), but if it's short enough you just don't need to care, indeed.

I don't work with this myself, but my understanding is that you only start ripping this out when you are positive every wavelength from every customer actually has coherent detection, and that could take a while. :-) Obviously this will differ from site to site, too; I would assume new ones don't care unless there's a lot of legacy involved.

Alien wdm isn't common in long haul systems though, if so the provider typically provides the transponder. Usually customers use regular grey (1310nm 10G-LR) to the provider and the provider turns that into something compatible with their dwdm system.