It is okay to set the value by way of a property node, but it does execute much slower than by local variable. To set a value or two, you won't notice much of a difference, but if you were updating a value in a very fast loop you would.
There may be other reasons as well that local variable is better than a property node. No matter which you use, be wary of race conditions. When you can't control when a value is being written vs. read, you can get duplicate or stale values, or may miss a value. Or if you have two locations writing to the same local variable, they would essential compete as to which value sticks in the local variable.
One situation where the property node is better is if you were using the value(signalling) property. If you were using an event structure to detect when a value of a control changes, writing a value to that property node signals to the event structure that the value changed just as if the user had typed in a new value. The regular value property node does not signal the event structure, nor does the local variable.
