Is the rise time of the output of a logic IC or optocoupler independent of the rise time of the input?
For really slow input transitions there may be some jitter on the output. Try looking at schmitt triggers- They have built-in hysteresis that takes a slow input and creates a fast jitterless output. A more technical discussion is here. Adding one would also make the rise time frequency independent. An example part might be the CD40106B CMOS Hex Schmitt-Trigger Inverter. It has a similar voltage range to the 555, but double check to make sure it works for your application.