How much of humor in a formal academic writing is considered as disrespect? In a formal academic writing (in the everyday sense), is it appropriate to use phrases such as "step out on" to titillate the reader? 

Since I am not a native English speaker, and since I have not yet accurately catched the general range of humor in the US, I wonder if using such phrases would be instead considered as a disrespect?
Your first priority should be to convey your ideas to the reader. Given that I, and many others, have absolutely no idea what "step out on" means, I would suggest that you avoid the phrase. On the other hand, I think humour is occasionally acceptable, but it needs to be timeless humour.