Hysterisis - How do you choose a good value for R3?
I would not say it is very important, but it's a degree of freedom. 

Looking out from the comparator input you see a Thevenin equivalent source resistance of R1||R2||R3. So there will be an error due to bias current of Ib *(R1||R2||R3). Bias current varies with temperature etc. Even if the comparator is a CMOS type you probably don't want to go too high lest the input be noise sensitive or sensitive to PCB leakage or less stable. It's also affected by the precision you wish to achieve. 

If your input is actually applied to R1 rather than the inverting input of the comparator, then the source impedance of your input signal is added to R1, and some changing current is injected into the input which may affect it. 

If your comparator must operate very quickly you will want to use relatively low value resistors because stray capacitance and input capacitance of the comparator will affect the performance. If you're dealing in in nanoseconds you probably want R1||R2||R3 to be a few K at most. 

Another consideration is that often R3 will turn out to be much higher in value than R1 or R2, especially if you are only adding a taste of hysteresis so you don't want R3 to be too high or it may become more expensive or harder to buy. 

On the low side you are consuming power from the supply due to the resistors which may be  a consideration and the output must not be loaded too much by R3 + (R1||R2). 

So a reasonable range for R3 might turn out to be 10K to 10M (no doubt with exceptions in extreme  situations), depending on the hysteresis required etc. You can use this degree of freedom to optimize power consumption (higher end), accuracy (pick precise ratios using standard series resistors and minimize bias current and leakage effects) etc. 

Sometimes it's better to pick a value for R3 (say 1M or 100K) and work with the other values. Whatever you come up with, once you have design resistance values, you can scale the three values to get closer to standard value, values you happen to stock etc.