This is an age old argument that cannot, technically, ever be satisfied by a definitive answer.

There is of course a possibility of the fuel in the inlet runners igniting when the spark from a wasted spark system fires on a dead cylinder that still has an intake valve open.

The problem with the argument is the fact that it is just as much a danger on a system not using nitrous, especially one running batch fire injection, as that is essentially the same thing. The NOS is rather irrelevant given its inflamability at the intake runner stage.

I know of only 3 cars that have ever had an issue this way, and i look after well over 100 Nitrous equipped cars, a good 50% of them running WS ignition systems. An example many of you on here will recognise is Lee Catermole (TiB) How long has he succesfully ran Nos and Wasted spark with 500bhp and a healthy dose of NOS? Yes, years. He is one of many many thousands i assure you all.

All we can do is view it as another very small but real risk, just one more of many we run when we modify our cars for more performance.