Sigh... again with the percentile thing.
Okay, I can see your point if you just use the basic mishap table. But really, the meat of the book is the three tables in the back that divide mishaps into three categories. Those haves stretches that aren't in 5% increments.
Second, this is a table. When we are talking about straightforward resolution rolls, yes, please do use d20 if possible. But IMO tables really don't have a compelling reason to use a d20.
'nuff said.