I've always asked it in this sort of style and don't remember anyone ever having a problem with it, and as the author points out, it's critical for your evaluation of the risk of the startup, which is necessary for you to price yourself, set your expectations, etc.

Asking this shows interest in the company, asking "How long can you guarantee that you can pay my salary?" sends a very bad signal of among other things risk aversion. The employees of startup have to have a healthy appetite for risk....