The short answer is it depends! And I suppose that's not too useful. Here are some things to consider though:
When you engage this type of calculator you need to consider what the goal is of using such a tool. If you are an industry professional, for example, then you would need a deeper working knowledge of the products and the aims of the client whereas if you are a Senior just getting to grips with idea of choosing a reverse mortgage, then this type of device can offer some ball park figures.
For the most part the online calculators really give you outlines of the amount of equity you can release and your eligibility. Once this is established I would strongly suggest that you engage the services of reverse mortgage professional, and I do stress the 'reverse' part as opposed to a regular mortgage consultant because the elements of these products are very different.
The advisors who work the senior end of the market tend to handle their clients differently too, but that's a side point. Should you or should you not bother with an online calculator? If I was just starting out and needed some idea if I qualified and what benefit I might get out of it, then go right ahead - use an online reverse mortgage calculator you've got nothing to lose. If you need in depth analysis and information relating to HUD mortgage products then give your local guy a call. It will save a ton of time and might even save you a small fortune.