To answer your question, my opinion at this point is, since you've spent the money, continue to use the unit and don't worry. In 35 years of doing this type of work, I have experienced thousands of this type of service call where the unit performed for many more years without needing another charge. I still service some units over 30 years old where the customer refuses to have them replaced, that never need a re-charge

Personally, I don't think 10 psi of refrigerant loss on an older unit is that big a loss. It is only a short shot of gas to bring it back up to full charge. I certainly am not a fan of dye testing, especially on that small of a drop in charge. I would have performed a halide or electronic leak detector search if I suspected a leak on that unit. That would pick up the most minute leak. That's not to say the technician was less than adequate, it just means his technique was different than mine. He also has most likely seen many units where that is all that was needed.

FYI, A dirty coil could cause the pressure to drop that much. Also, with the amount of recycled copper in use, the refrigerant can very likely leak through the pours of the copper tubing. Units made in the past 15 years or so are more susceptible to that since the copper is not as dense as that used in much older units. I've tested that myself by weighing equal pieces of copper tubing, the older is much heavier. Also, more aluminum is used in the refrigerant circuit than was on older units. A proper charge of oil and refrigerant helps to minimize that.

What you can do now, to help you determine if you have a real problem (depending on your DIY skills)....look for evidence of oil spots anywhere along the refrigerant lines, especially at joints. If they have any age to them, a small amount of dirt would stick there. Also, right now would be a good time to get 2 thermometers and at the plenum, check the return air and the supply air temperatures. You should see somewhere between a 15-20 degree temperature difference. Make sure the filter is clean. Make sure there is condensation coming out the drain tube. Also, at the outside unit, check the air coming out and make sure it is hotter than the outside air. These are all indicators the unit is most likely cooling fine. A precise determination would of course require more trained detail. If you start seeing a marked change a few weeks from now, from what you observe now, then you can suspect a leak. You don't have to do this every day, but as often to satisfy your curiosity. Also, if you start seeing ice form on the lines you can suspect a lower charge. There are other things that could cause that but it is an indicator of a low charge.

All that said, it doesn't mean you don't have a tiny leak however, you now have some tools to help you determine whether it is a possibility or not. Look at it this way, most likely it will be some time before you need another recharge. Something else will most likely go wrong. You now have an early warning that your unit is aging and will need replacement sooner or later. You have bought some time to prepare for that day!!!