As long as you catch the run off quickly, it is a good solid way to test the soil ph. If you wait too long, then it will be a mis-reading skewed by the high levels of freshly ph'ed water. Since you test when watering/feeding, the soil should be fairly dry in the first inch or so of the container. Make sure whenever you do water or feed though, you water "through" the container - My rule of thumb is to allow around 1/4-1/2 the container volume to flow through the container. This helps flush any nasties and makes sure all areas of the pot are watered and aereated. Its from this runoff (the very beginning) that you get your sample to test ph.

Soil ph should be anywhere from 6.3 to 6.8 , which is slightly higher than you are used to with the hydro setup - shoot for 6.5 and its all good

Good luck!