Since the fuel is injected directly in the combustion chamber, NO amount of fuel additives, cleaning agents, etc will ever touch the valves. The injectors will stay cleaner but the valves will not. The deposits build up as a by-product of blowby crankcase gases being sent back into the intake via the PCV system. Crankcase gases have vaporized oil that condenses and collects on the insides of the intake ducting, intake plenum, manifold and intake valves. Preventing those vapors from condensing on the components is key to stopping this type of deposit build up on the intake valves.
The installation of a quality
oil catch can can, and maintaining it, will aid tremendously in this effort. As for knowing what, if any, deposits are actually built up on your valves, the BEST method for determining that is to actually look...either by directly removing the intake components or using a remote camera probe. Only then can you determine what course to take.
As for cleaning, there are several methods:
An intake cleaning like "BG" services or dealer offered services. They can work, but they can be expensive (over $100).
Self-clean every 10K miles or so with products as SeaFoam, etc. The drawback here is that unless you know the amount of deposits on the valves, you won't really know if it actually worked. I have quickly become a fan of CRC intake/turbo cleaner as I just used it on my vehicle as a 'test'. I have used SeaFoam and other products on other conventionally combusted carts I have and had good results. But I have switched to CRC per results and video below.
Removal of intake components and walnut-shell blasting, if necessary. It does the job, can be somewhat time consuming, and needs the right tools. You also must be very clean and ensure to block off valves you are not cleaning to prevent shells from entering cylinders. I have no experience with soda blasting these deposits. So if you are comfortable wrenching on your vehicle, this would be a way to do it.
What sold me on CRS was this video. It does appear that CRC is formulated to dissolve carbon deposits and it does apparently work quite well. I can get a can (on sale) for around $10 and then get another $1 off military discount at O'Reilly, AutoZone, etc...or purchase on AMazon.
This approach can cost less but also be time consuming, and does not take specialized tools (Like a blaster, walnut shells, masks, gear, etc) other than the cleaning agent, a couple of old toothbrushes, and a couple of picks. Though I did just use the intake method with CRC a few days ago, and I am checking mpg etc, I expect I will soon just open up the plenum and manifold and do it this way:
Once it is clean, and a
catch can is installed, I believe that carbon intake valve deposits will be pretty much under control. And 'cleaning' will only be required on a preventative basis. If you, like me, have a 6 cyl 3.8, and you are going to do plug replacement, that would be a good time to take it a step further and evaluate your intake valves for deposits and clean them at that time as you'll have the plenum off do do the
plugs. And it's easier to clean if you, in fact, remove the
plugs.