Is Spiky Memory "Healthy" for App?

My primary worry in these situations would be fragmentation. If the chunks are all the same size though, you should be fine (and looking at your graph, the peaks appear to be completely level, so I think that's the case).

You will be paying allocation costs, but as Ole says, if your app is performing well enough already, there's not much point in trying to optimize that.

It is neither risky nor necessarily bad practice. Allocating and freeing memory takes time, so doing it very frequently vs. doing it once and re-using the allocated memory is a trade-off between memory usage efficiency (using the lowest amount of memory at every single moment) and performance.

If the performance of your app doesn't suffer at the moment, you have probably made the correct choice regarding this tradeoff for your app.

Generally speaking, using 30 MB of memory is quite a large amount for older devices (iPhone 3G and older). You cannot be sure that your app has that much memory available so be prepared to received memory warnings. If your app cannot reduce its memory usage when it receives a memory warning, the OS might kill it.