There has been a lot of conern over this issue. DIAC looks at a range of things when looking at whether the applicant will be a burden on the social security system, one of them being the sponsor's income. If you've received full benefits of any government allowance or payment for an extended period, then you would probably be asked for an AoS. However, you have only gotten part payments and have another source of income, so I don't think you need to worry too much. They will also consider the applicant's past educational and work history and level of English to determine whether an AoS in needed.
All you can do is get the application together and hand it. Then it's up to the department to tell you if and when they need an AoS. And, if it turns out you do need an AoS, it's by no means considered a red mark on you as a sponsor and it won't affect the validity of the application simply because it was asked for.