Pieces still matter....The most important stat, however, is your stops (delivery + pickup). Once you learn what your stop count should be for the route based on the center's planned day (deliveries as well as pickups), along with your route's expected mileage, you'll begin to get a basic idea of what a scratch day should look like, and may begin to see how you may approach it....or if the route really sucks.
One doesn't have to break their necks or kill others in car crashes to try and get bonus, and cheating may be just as rampant in non-bonus centers as well; it has a higher profile in bonus centers because of the economic reward that may come with it (I've worked in a bonus and a non-bonus center). If a driver is proficient in their delivery methods they should have a good SPORH; if one day they have so many airs they must do a straight air run, thus racking up extra miles as well as stops, they may get bonus that day. Same thing if they have a typical day, but one day the widget factory sends out 150 more widgets than normal, affecting their piece count similarly; they may get bonus that day. Find out what your center's planned day is, find out the target SPORH for your route, and find your route's expected mileage. Then you will know what's expected of you, and realize when bonus may be coming.
Word of advice: don't "seek" bonus: run the route the way it is supposed to, keep your SPORH up. Don't try to cheat the system, because they will find out. Do a fair day's work for a fair day's pay and bonus may find you.