measuring content of soil is never precise with a runoff test nor a slurry. it can be useful, if you know the baseline for "normal." normal will depend on several variables, so it may be a bit different for you than others in subtle ways.
depending on how much runoff you have, the ppm could "look" higher than it really is.
in a solless grow, you ensure 10-15% runoff and this counteracts any evaporation that occured between irrigations, which in turn maintains a more consistent PPM level over time - avoids buildup if done properly.
you probably don't want as much runoff with a soil grow, as this will only waste your soil amendments and such. But, expect your runoff to be higher than the actual ppm of what is in solution in the substrate. between irrigations, ppms will rise a bit -- more so for a small plant in a large pot because a greater proportion of water loss is through evaporation. And, as you gain experience, you'll see what a "normal/tolerable" range is that has good results and avoids any issues.
the plant can handle some ebb and flow, of course... don't think you have to be nuts about it. you'll find that balance relative to how you grow... size of plants, size of pots, rate of evaporation relative to how much is drunk between irrigations etc... should impact how you should approach it. observe and react... adjust... do better next time. some plants can be oddballs, but 80-90% want the same things. i've grown 7-8 strains at once with one formula from a single rez... not a problem with a good balance of nutes.