Negative emg - diagnosed with ulnar neuropathy. is surgery still beneficial as a preventative measure (to avoid muscle wasting) at this stage?

It depends : If the ulnar neuropathy was demonstrated from an imaging study showing a compression or other specific cause surgery might be beneficial but the lack of correlation to the EMG/NCV is bothersome. Discuss further with your doctor about the benefits of surgery and the current diagnosis before proceeding would be a suggestion.

-ve EMG is OK: EMG/NCS have quite a significant false negative rate for ulnar neuropathy at the elbow so the diagnosis remains possible but it could be argued that if there is no measurable nerve damage and no wasting or weakness now, it would be reasonable to try less aggressive measures first. Conservatively treated cases seem to do more or less as well as surgically treated ones overall.