Does ACM have a style guide for authors?
You should take a look at the format that the ACM template gives to you, but there is no straight answer for an specific format. Try to focus more on the content (like they have advise you on one comment) that in the format. Actually I have seen in some ACM published papers that they put things like (considering your example):


In Equation 5... or
In Eq. 5 ... or
(put the variant you think here)


Consider that is one of your least worries, if you had some problem with the format the reviewers will point to that, but if the content is good is what really counts.

Good luck!