Is it useful in C# to apply DeMorgan's theorem to manually optimize boolean expressions in conditional statements (e.g. if conditions)

On processors this fast, it's virtually impossible for rearranging boolean expressions to make any actual difference in speed. And the C# compiler is very smart, it will optimize it as well. Optimize for readability and clarity!

Your first goal should be to optimize such statements for developer comprehension and ease of maintenance.

DeMorgan's theorem can be a useful tool for this.

The optimization in the JIT, in its current form, does not (from what I've read) optimize this for you. If you need to optimize it, you would need to still take this into account.

That being said, this is a fairly small micro-optimization. In general, I'd prefer to write your "non-trivial boolean expressions" in a more expressive form so they are easier to understand. To me, this is more valuable than any very small optimization you'll get from applying deMorgan's theorem.