The database for the location fields uses United States, which would be the current preferred name to use in the location fields. But USA and United States of America are considered just as acceptable.
I usually use United States of America in the biography. You are free to use your own preferred version. Just be sure to match it to the temporal name of the location - e.g., no United States/USA/United States of America before July 4, 1776 (later for states 14-50, some territories and possessions, etc.).
Most policies on WikiTree are somewhat loose in interpretation and usage, in my opinion, which gives us lots of room for lively debates!!