Question: I work with low-power DC voltage regulators. I am already aware of the formula to calculate the size of smoothing capacitor(s). This can be an iterative process of testing one size with a scope and then using a larger size or adding more until the scope shows acceptable (very low) levels of ripple and noise.

Besides the cost of the capacitors, is there any tradeoff to rounding up (a lot) and just using a very large capacitor(s) rather than trying to calibrate the sizing to "just enough" but not more than that?

Answer: 
As far as caps go, there are two competing requirements:long-term (ripple) and instantaneous (spike). A big electrolytic can give you the former but not the latter. Generally you parallel your large electrolytic with a smaller 0.1uF capable of supplying that instantaneous spike whilst the electrolytic lumbers into action. Or the 0.1uF may be for local decoupling to stabilise that regulator. If the specified capacitor is actually 0.1uF or smaller, then the intention of the capacitor is to supply small amounts of charge very fast. Do not replace this with a bigger electrolytic - that's definitely a case where larger is worse not better.

Going past that, you'll have to tell us what kind of regulators you're dealing with. If it's just a basic linear regulator then it doesn't really matter. If you have a switching regulator though, the capacitor will affect the resonant frequency of the switcher, so be very careful there.
From Andy akas comment:

If the supply you are using has [specific output capacitor requirements](http://www.ti.com/product/lm1117?keyMatch=lm1117&tisearch=Search-EN-Everything), then make sure you follow them. For all these types of regulator linked (LDO), there is usually a minimum capacitance only. (search the datasheet for ESR).

If you are using a switch-mode regulator, then the output capacitor (in current mode controllers) determines the output [pole and zero](https://en.wikipedia.org/wiki/Pole%E2%80%93zero_plot). In voltage mode converters, it forms a resonant circuit with the output inductor. In both cases, we must provide [loop compensation](http://www.linear.com/docs/46311) and that is partly determined by the value of the output capacitor(s).

(Note: I am aware that using ceramics on the output of a current mode device requires other techniques to provide an output zero as a ceramic capacitor zero is too high in frequency to be useful).

These capacitor(s) must be carefully [chosen](http://www.linear.com/docs/4165); changing these values requires re-assessing the loop compensation components, or it is quite possible loop instability can result.

This re-assessment may also reduce the loop bandwidth of the supply, reducing transient performance.