BDL and KY
Yes, I think there are two different kinds of rolled up cakes we're talking about here.
1. Some have the "filling" baked in, like the one i used to make with the buttery, nutty, condensed-milky "filling" baked under the cake batter. In that case, you use the towel or paper like rolling sushi (I guess, since I never made sushi). I suspect that's the original cake mentioned in the thread, though without the recipe i can't be sure. Once the cake is rolled, it just stays rolled and the filling is already in it since it was under the batter.
2. The other applies to the actual "jelly roll" or "buche de noel" etc. If you have the kind of cake batter that remains soft and pliable and if you haven't overbaked it so it got dry, and if you don't try to roll it too tight, you can probably get away with letting it cool flat and then filling and rolling up and it won't crack. But for most of us, the foolproof way to go is to roll the towel up INSIDE the cake, (where the filling will go), and let it cool rolled up. The cake is much more pliable when warm. Then when you go to fill it, with ganache, jelly, whipped cream or whatever you like, you unroll it, leaving the towel underneath, spread the filling on the semi-curled cake (cooling it retains some of its curled shape) and then use the same towel to help it roll up like sushi.