Although the 566 function generator is cute with both triangle and square wave output, it isn't nearly as versatile as the 555/556. There have probably been more DIFFERENT types of circuits designed around the 555 that there have been around the op amp.
You can't ever get exactly 50% duty cycles from a 555 without some external tricks (like the binary divider at the output). But for most purposes, especially if you're varying the frequency, setting Ra to a value of 1K ohm and then selecting the timing capacitor to keep Rb at least 10 times higher than Ra and finally selecting Rb for the desired output frequency will normally give you close enough to a square wave for all but the most demanding applications.
Note that Signetics does the duty cycle "upside down" in their equations. For the 555 output, a 75% duty cycle is high 1/4 the time and low 3/4 the time, just the opposite of what you're normally taught. Not sure why they did that ... it was always confusing to me.
Dean