I agree with the comment from Aadam re: medical device. Approach this one with caution. Perhaps get input from your expected userbase, too?
What I personally would suggest is on startup, offer the OPTION to enable automatic update checking and a checkbox to "never ask this again". This way you put the choice in the hands of the end user (or their IT department), while still offering the automatic update functionality. You could also in the same dialog offer some simple options to choose update check frequency, and/or action to take on update available.