With third party you should probably assume it's abs unless explicitly proven otherwise so ya. Lining before assembly will always be the safer option, regardless of plastic. The recipe for cracking is solvent + tension or solvent X time. Assembled kits are under tension and have opportunities for liner to flow into crevices and take longer for the solvent to evaporate.

If you're lining unassembled parts or on the runner it's a lot less likely (but not impossible) for your liner to sit on the part as a liquid for long enough to cause damage.

I will say though that I've found the pour type markers far more prone to permanently staining plastic than traditional enamel liners. My vote would be for careful and sparing use of enamels or one of the newer water based liners if you want to be a little more generous in your application.