[Yes it can be straight all the way down the end bar. They are harder to pry apart, but they also swing less in transport.]
Bingo, that is the answer the question was asking for. They are tapered to reduce propolizing so the frames are easier to remove, thus resulting in less damaged equipment. Note some plans don't include rounding the physical thickness of the top taper in alternating corners, this makes them even easier to separate.
-------- <-Top Bar
[...the bee space between TOP BARs is also reduced...]
[...top bar width must be reduced to 15/16"...]
Dave points out a very important precaution. Last year I cut my top bars just a 1/16" too wide, 1-1/8", and beespace was tight between frames and nearly non-existant along the hive body walls. The bees will propolize/burr comb this area, but more importantly it becomes unusable space and causes crowding, which in turn can promote swarming.
I have since gone to 1" wide top bars and I use this size because when you notch them for the end bar shoulders/ears you have a deeper and studier grove and fit. End bars are 1-1/4" wide and maintain the standard 3/4" top bar notch [so you can still buy commerical and cut/shave the 1/4"]. When you do the math and fudge the numbers for a little working propolis, it works out right about a healthy 3/8 " beespace.
If you are looking to mod commerically bought frames, I'd say you can tablesaw [for straightness] a skinny 1/8" off each side of the top bars carefully so not to damage the endbar groves. You can plane 1/8" off each outside edge of the endbar, but I don't plane very level so I perfer to saw them also [just watch your fingers because mechanical finger damage takes months, if ever, to heal]. When sawing be sure to use a sharp 40+ tooth blade for a nice finish and better control when pushing through the cut.
[and PS. if you need to move/ship a hive with tapered endbars, push pin thumb tacks in the bottom of the endbars help prevent too much movement]
There is always more than one way to skin a cat, that's of course if you're into eating cats.