Hi, Bob. First, are you talking about inside or outside corners? If outside corners, are they actually 90 degrees? Check this by putting a bevel gauge on the corner to "copy" the angle and then read the angle on your kid's plastic protractor. It's not unusual to find a corner that actually measures 88 or 91 degrees or something and you can compensate on your trim saw. Put the part of the molding that goes against the wall against the fence of your saw.
For inside corners, a coped joint is actually easier since it will allow for an off angle and, again, a butt joint is easier than a miter joint to match up. Crown has the angle of the molding to consider, which you don't have to worry about (and that's why the Delta chart doesn't add up to 90).