mc34063 is a cheap switching regulator with minimal input and output protections, which can be damaged easily, too easily.
In addition to that, it's also relatively low efficiency, with high quiescent currents, requires a lot of extra components etc..
There are far better switching regulator chips out there these days, just a bit more expensive but in some cases with the added benefit of saving you money by allowing you to use smaller inductors and ceramic capacitors and resistors.
You would design your circuit for the maximum current it would have to give to devices plus some margin for safety ... ex if your circuit won't use more than 100mA, maybe design it for 125 mA max. Usually, the more you tweak it to be able to do higher currents, you lose efficiency at low currents.
For very small current amounts, sometimes it may make more sense to use LDOs, as their efficiency won't be much different than the most a mc34063 would be capable of (maybe 75%-80% with perfectly picked parts)