Why wouldn't have an offset account in both circumstances?
Say you had a negatively geared property, what happens if you get a large windfall which you intend to use the funds to buy a property with, but the one you are after isnt available yet. The funds need to be parked somewhere - where better than an offset account?
Because that would reduce your interest. If you reduce your interest you will save 5% (interest rate). But if you don't you can get back 30% (your marginal tax rate) of the taxable loss as a tax refund (if there is a taxable loss that is and if you have already paid enough tax).
If your strategy is to "stay negatively" geared then you don't want to reduce your interest.
Personally I don't use negative gearing. Theoretically negative gearing can work but in reality it doesn't always since capital growth is not guaranteed and is only an assumption.
Andrew