Deterministic and noiseless are two different things.
If you want the model to run the exact same way every time you run the model (in other words, running the model deterministically), you need to set the seed.
If you want the approximate the output of X in a noiseless manner with neurons, that’s impossible. The noise of a representation. If you want to approximate the output of X, in a noiseless manner, bypassing neurons, you can use the nengo.Direct() neuron to simulate the population in direct mode.