i didnt use an agent for our visa 190 application last year. our case was straightforward enough for 2 adults (unmarried- this was not even questioned) and 4 children. i was meticulous with gathering evidence to support our application and left nothing to chance, for example, one employer didn't answer, when we emailed for details of the employment so we didn't use the points even though it would of given us an extra 5 points, i didn't take the risk.
as others have pointed out for a straighforward case, i say go ahead yourself, if your unsure go the MARA route