I believe that it's a university extension program which means it'll only count 0-5 points towards your aggregate (instead of 0-50), ie it can't be in the top 4. It's its own subject so it wouldn't be affected by any good or bad marks in the other VCE subjects
Doing the step-up program would be good for the future as I believe you get credit for it in university, but I'd only take it if you were either really confident in your 4 subjects (as they'd all be in the top 4), or if you took 5 subjects so wouldn't have to rely on all your subjects doing well.
Good luck in your decision and let us know what you choose
Sorry, to clarify... I think when the OP says that it counts as 10% - it means the program is ALWAYS an increment (i.e., considered a 5th or 6th subject), but instead of funcitoning as a uni extension subject does with marks from 0-5, it takes your average study score and gives you 10% of that as a 5th subject.
So what I mean when I said it's like a free 40...
If your average study score was 40, then you'd have your top 4, and then you'd have your 5th subject be a 4... which would be like having 5 subjects of 40.
(I could definitely be wrong - but the 10% increment thing is what you get when you do a Cert 3 or 4 in retail or something else that counts to your ATAR, because there are no marks awarded for that certificate in the same way as uni extension!)