Depends on where you live and where you want branching to start. Cherimoyas go briefly dormant around April in most temperate climate zones (northern hemisphere, of course!), dropping their leaves and then pushing new ones. So if you will still have some warm weather you might get some new growth by topping the whip now. On the other hand, that new growth will be tender and more subject to frost damage, if you get any frost. I'd be inclined to plant and stake the tree as-is. When the leaves fall around April I'd top it at about 24"-36" and let branches begin to form from there.

Some kind of deciduous stone fruit, like a cherry. If you are bothered by the fallen petals, you really won't like it if it ever has a good crop of fruit. I was once approached - while gazing upon some Japanese flowering cherry street trees of a locally rare variety - by a duffer with a flat-top haircut and an artificial lawn and no trees or shrubs who hoped I was there to cut the trees down. You see, the fallen petals made "an awful mess" and he just had to get his blower out each year and blow them away. In Japan there is a tradition of setting out blankets in front of flowering cherry trees, having a picnic and gazing upon the falling petals etc. To become comfortable with the situation you either remove the tree to accommodate your existing point of view or change the way you think about it and keep the tree.

Hawaii has so many microclimates--the weather gets cooler the higher the altitude and depending on the tradewinds, proximity to the sea and other factors. While of course I'm nowhere near getting frost, cherimoyas have the potential to grow very well up in the 3,000-5,000 ft. range, while they won't thrive in the lowlands. It is true that the ilama might grow in mountain climates elsewhere, but elevation isn't all relative and the Hawaii growing guide I've read lists it as strictly for the warmer lowlands here. Often the information is quite confusing. Mango Kush, I have eaten a few local mangosteen grown on the island, but the trees are still very rare here (and they definitely wouldn't grow on our land). I might do a writeup with pictures on my project, and if I do I'll post a link to it here.

You said it yourself. It's a tree. Let it be a tree! It wants to grow!