Folks:
This is correct; it is customary to located the 1-ball and the 5-ball to the corners of the rack, for the following reasons:
1. These are the two brightest-colored balls in the rack, no question. Since the standard opening break shot is a safety by clipping one of these two balls and sending the cue ball back up table, these two colors -- yellow and orange -- are the easiest to see out of the conglomeration of colors in the rack. The breaker has an easier time aiming at these two particular balls, and thus, it is considered a courtesy to the breaker to locate these two balls in the corners like this.
2. The rules on the opening break are simple, two object balls plus the cue ball must touch a rail. In the opening break, it is easier for the referee to verify this requirement by having the two brightest-colored balls *be* those object balls that he/she is watching carefully to verify rail contact is made. Thus, it is a courtesy to the referee, as well.
3. From the breaker's point of view (as viewed from the head of the table), the 1-ball and the 5-ball are located in such a way so as to be in "numerically-ascending" order when reading the back row from left to right. So the 1-ball is located in the left-most corner, and the 5-ball is located in the right-most corner (again, as viewed from the head of the table, reading the back row from left to right, as you are reading this sentence right now). Obviously, the racker reverses this sense when racking, since he/she is viewing the rack from the opposite side.
All this is discussed here:
http://en.wikipedia.org/wiki/Straight_pool
Hope this is helpful!
-Sean