If you are going to add insulation to your attic, do it after you tromp through your attic to install the radiant barrier. Insulation depends upon being fluffed up, since the more air pockets you have, the greater the insulation effect.
Every time you step down on insulation, you are reducing its effectiveness (if not totally destroying it). Crawling into the lower tight spaces will inevitably squish some insulation. The goal is to make your house MORE efficient, not less.
Some companies give you discounts in order to push more insulation. Whether you need it or not is irrelevant, because once the hired hands stomp down the existing insulation, you will need to replace what was squished. Crawling around in an attic without touching insulation is a very difficult thing to do, this is why most companies want you to buy more insulation after they put in the radiant barrier.
Once the insulation reaches 6 inches, every additional inch of insulation helps a fraction less than the inch before. What this means is that a house which already has the 2x6 frames covered, really needs to have help with the ventilation or radiant barrier, before getting more insulation.
Now. I'm here in Texas, so all of my information is biased towards stopping the heat. If your climate is mostly cold weather, you won't get near the benefit out of a radiant barrier that someone down here will. As a matter of fact, stacking the insulation as high as you can will help to retain the heat. Since insulation is cheap and easy to install, it is worth the $150 to go buy 20 bales and through it around your attic.
Lowes and Home depot will both loan you the equipment (with a deposit, in case you destroy it) if you buy 20 bales (roughly $7 a bales for cellulose). So paying a radiant barrier company $500 in order to let them hides their tracks with $150 worth of insulation doesn't feel worth it.