If you talk/watch to pro-gamers, you'll find that this is not a decision that is made as part of the game. The closest they get to this decision is whether or not to build one or two refineries early in the game or how soon they need to get their gas running on expansions. The logic behind these two decisions has to do with what units they want to build. Overall, the resources at each base are fairly balanced. Once you're past the first few minutes after you get an expansion running, your gas should be saturated as well unless you absolutely need 0 gas for your build (which is unlikely since an all minerals build is easily countered).
If you plan on building Tier 2+ units soon, you'll typically need more gas. This will cause you to take an early geyser or grab the gas when you expand. Otherwise, it makes more sense from a financial perspective to keep the workers that would otherwise have built/mined the refinery on minerals to get those additional 40 minerals per second apiece (especially if you're going for an early expansion). This isn't a decision made based on "what should I stockpile more", it's a decision about what makes sense economically.
As you've noted, even pro-gamers are unbalanced in what they have saved up. Most of the time, this isn't because they've decided to stockpile one resource (although this is occasionally the case in the early game because they're saving for a timing push or an early expo). In fact, the contrary is usually true: The resource they'd prefer to have stockpiled is the one they don't have. Why? Because the most effective players spend resources as soon as they get them.
Any unspent resource is not contributing to your army, gathering, or technology improvements. In terms of helping you win the game, unspent resources serve as much benefit as an IOU. So, as you get enough minerals or gas to do whatever you need to do next, you spend it immediately. If you follow this exactly, you'll never be above a few hundred units of the resource you need most, and it will appear as if you are stockpiling the other resource (since you need it less often).