I am working with a '90 Toyota tercel 1.5L sohc 4cyl.
and for reasons unknown to me, every month or so, the #1cyl
spark plug needs changing due to broken porcelin surrounding the electrode which in turn causes the #1cyl. to miss-fire. I undrestand that carbon build-up could eventually reach a spark plugvalve with the piston at tdc; but I dont think that is the case since I have not been fighting any pining problems or dieseling.
The plug comes out displaying a color that looks good to me so I dont think it's burning lean iether.

You're using a torque wrench and a cushioned socket to install the replacements.. right?

and you can get a good angle on the wrench, and you are holding the head of the wrench not just whaling on the tail? That's why I prefer a speedbrace to be honest...

Starting to sound like a detonation problem.  Your PCM may not "hear" destructive detonation.
  
Keep in mind that the "porcelain" is one piece and it is conceivable that a crack could be started from improper installation, I have not seen that except in one extreme case where the jerk used an impact wrench on a Ford taper seat.

You stated that "the plug comes out displaying a color---etc."???  
How can you see a "normal" plug if there is enough detonation to "crack the porcelain"?

As you seem to already know, it's time for some basic diagnostics.  Something is definitely NOT "normal"!

If the porcelain cracks in the chamber always on the one cylinder but the ground strap has never been bent from impact, it is detonation.

If the leads are the wrong way around on a 4 banger, it means that 2 cylinders are at least 180 deg out and the misfire will be obvious, and most likely spectacular with great flames erupting from the induction or exhaust and less than 50% power output if it runs at all.

The cylinder in doubt has higher compression, gets a higher charge density, has more advanced timing, has a built in glow plug or is running leaner than the rest.

The reasons for one or more of the above can be one or more from a large and varied list, from faulty ignition system to faulty fuel system to worn camshaft.

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