WARNING: THIS IS ABOUT RELOADING, AN INHERENTLY RISKY ACTIVITY; PROCEED AT YOUR OWN RISK. THE COMMENTS BELOW RELATE ONLY TO LOADING HANDGUN CARTRIDGES.
This gets at an issue that causes much confusion in reloading: what is the meaning of a "starting load"?
The answer: it varies. The method by which a great many manuals calculate a starting load is simple. First, they test to find the max load. Then they subtract 10% and call that the starting load. For starting loads calculated this way, there is nothing magic about the published starting load. It is not a minimum load, just a starting load.
However, there are some powders that do not respond well to being downloaded. Ignition becomes erratic, speeds get unpredictable, residue gets really dirty, brass doesn't seal, the load may even fail to completely burn, etc. Powders that are especially intolerant of downloading include many of the slower handgun powders.
So how do you know whether a published starting load is also a minimum load? Well, many manuals will often have a note about particular powders (such as H110) that should not be downloaded beyond a fairly high point. For the rest, consider checking a manual, such as Hornady, that lists loads other than just max and max-minus-10%.
You can also look at other applications of the powder in question. Particularly for something like .357, look at the .38 special loads. If a powder is well-suited to running at ~15,000 PSI in a .38 special load, it will probably run fine more than 10% below the 35,000 PSI .357 magnum peak! Similarly, someone loading lite for 10mm can look at .40 loads. 9mm light loaders can get some sense from .380 loads. And so on.
Remember, there is risk in going below the published loads, but the risk is very different than going above them. Barring an undetected squib/stuck bullet situation, the risk of a catastrophic problem in a low-loaded handgun cartridge is virtually zero. The action may not cycle. You may get erratic velocities and lousy accuracy. The gun may get filthy after just a couple of shots. You might get a failure to seal the chamber, which could perhaps, over extended and repeated firing, potentially cause some erosion or roughening of part of the chamber. But the gun is not going to blow up just because you dropped .5 grains less of a fast-burning powder in the case than what a manual listed as a start load, particularly if their start load is just 90% of max.
As Richard Feynman once said, "There's plenty of room at the bottom."