Well, they're just that... guidelines. We're not gonna raid you if you use a suffix instead.
However, using a prefix puts it in line with other topics that use prefixes (otherwise there'd be a 50/50 chance for either, not taking personal bias into account as individual people may prefer one over the other in general), which makes it easier to skim through a list of topics to find ones that haven't been solved.
On top of that, a suffix likely won't be visible in the new posts list on the index page, while a prefix will.