According to 37 CFR 1.33b, correspondence relating to a patent application should be signed by either a patent practitioner or the inventor(s). Reading the section makes me think it is generally preferred that the practitioner sign, but it seems more natural and appropriate for the inventor to sign that he/she qualifies for small or micro entity status. Is one or the other preferred? Does it really even matter?
Only the new micro entity status involving "means testing" requires a separate form to be signed. Small entity status (all individuals and any company with less than 500 employees) is usually declared by checking a box on an Application Data Sheet form that would normally be signed by the practitioner.
It doesn't matter to the USPTO which allowed party signs the micro entity statement. However, as a practitioner, I have decided to rarely recommend it and never to sign them myself. I do not want to be the one certifying that the inventors all made less than a certain amount of money last year or otherwise made more than 50% of their income from an institution of higher leaning. And that none of them have filed more than four applications in their life.
The concern regardless of who signs is that it is a very new program and many things about it aren't 100% clear. How a spouse's income in a community property state figures into the equation is one issue. Another is what penalties might be imposed in a situation of unintentionally erroneously claiming micro entity. Does the patent get torn up? Has the practitioner committed inequitable conduct? Does the inventor say they misunderstood the criteria and it is all the agent or attorney's fault?