No, AFAIK, the only way to ensure a fixed-step time for your model is to pick a Fixed-Step Solver and specify the desired step-time. Please read the documentation section Choose a Solver to understand how the variable-step solver chooses the next time step based on the dynamics of your model.
However, if you need a certain block only to run at the specific rate, you should be able to get that to work by specifying the block sample time. Not all blocks have this option though - which ones are you using?
