When your team feels they need to start testing on various platforms is really dependent on your entire development philosophy and process. Most major studios already have these processes in place, and have specific milestone gates to pass, some of which will likely specifically require standalone builds on the platform(s) the game will be run on, other stages may require a simple demonstration of gameplay and art assets, which do not require this. Many independent developers will want to simply have the shortest iteration time possible for getting basic concepts proven out (much like this book had done up to this section), which is by far the most streamlined for simply testing in the editor in PIE on a PC or Mac. However, if you're reading this section at all, it means you have a need and interest in eventually running on devices, and the best advice here is, if at all possible, have those builds working from day one of development, and build, deploy, and test regularly. This is another aspect of development often left neglected, or dormant, for too long, and then when needed most and used, causes unforeseen problems. It's definitely easier to develop day to day without taking the time to make standalone builds (or even use the Play button); but the longer a team goes without doing these, the greater the risk that when they do, there's a problem, and those problems are typically ones that prevent release entirely. So build, deploy, and test as often as possible!