Rolling round is done with convertible notes. No valuation talks take place.

Convertible notes are loans that can be converted into equity if some predetermined event takes place (usually you raise another round of money or you get a buy out offer.)

You're not selling X shares at Y price. You're issuing a loan that may get converted into X shares. Every investor who gets these convertible notes usually will end up getting the same valuation in the future if the note converts to equity.

So with rolling round, you're just taking on debt. And you can take as much debt as you want to (unless you've added a clause in any of the notes that determines that you can't go over a certain amount of debt.)