This is an interesting question. I'd refer to texts by Fletcher and Rossing on the subject as they are world leading experts in the physics of musical instruments. I can only tell you what I've learned from guitar playing, specifically classical guitar.
You should know that the brightness of the note produced by the string depends on where you pluck it (I am referring only to guitar and not hammered or bowed strings). So it is hard to say that any spot is "sweet" since that is somewhat subjective. If you wanted warm tones, al la Wes Montgomery, you would be wise to pluck at the mid point as that will accentuate the fundamental and kill a lot of the harmonics. If you wanted a very twangy tone you would want to pluck near the bridge as that will excite a lot of high frequency overtones and with considerable amplitude. So it would seem like you do NOT want to stay in one spot if you want variety and control of your tone. But who's to say which of those tones is "sweet"?
There are other factors to consider. By experimenting with the attack placement one may sacrifice other qualities like sustain and volume. So there is a trade off and I am not sure anyone has really studied this is detail but if anyone has it would be in Fletcher and Rossing's texts (if not originated by them). Various schools of thought are split on this in the classical guitar world. I recall Pepe Romero's book teaching that the strings should always be plucked at the same spot, just behind the sound hole in the direction of the bridge (I am pretty sure it was Romero, but could be Parkening) and that any other variations on tone could be achieved by varying the attack angle of the nails, pressure applied to the string, etc. In other words his method supports the idea that one gets overall superior sound in one place and that other variety can be achieved by controlling the attack parameters. However, not everyone agrees with this. I have seem video of Julian Bream where he will sometimes pluck over the finger board. It is also quite customary for Flamenco guitarists to play fast runs by plucking closer to the bridge but this is not for tone as much as the string feels stiffer there making it easier to bounce the fingers off the string, in contrast to feeling like a loose rope or wet spaghetti when plucked further in. Based on this you can see that there are other factors than sound that contribute to these decisions.
I cannot say for sure that Romero is correct (or the comment I have attributed to Romero) but in my experience it sure seems to be. When the hand drapes over the strings the fingers will lie across them diagonally, each finger touching the string at a slightly different place. This very fact makes it impossible to achieve consistency in placement, especially when playing chords. Perhaps they are in a small enough neighborhood about the sweet spot that it doesn't matter. My opinion is that this is true and it is a standard part of classical guitar training. But I have not seen objective data on this and would deffer to that.
The above does not likely hold as much for the electric guitar. While the basic string physics is all the same the amplification, sustain, and to some degree the tone, can all be controlled by electronics. With just a plain amp you can still hear the variety of tones generated by pick placement but the other issues that exist in generating a reliable note on the classical just do not exist for the electric. At the extreme I would guess that a large enough effect rack could make attack precision an unnecessary skill.