TBH i wouldn't think its worth it. Access is so limited that you are only going to get a proportion of it and as someone who has cleaned up more valves than i care to remember you are really going to struggle to chip away at the burnt on coke manually, its absolutely rock hard to the extent that i have in the past removed valves and turned them in the lathe to get the crud off.
Go for a vapour type clean, its cheap, takes half an hour and will benefit the valve seats faces and exhaust ports etc as well as the tops of the pistons. I did mine that way about 20k miles ago and it put the orange engine light out that had baffled Audi themselves for 18 months and half a dozen visits before that.
Paddy, I know a breaker that stripped a vehicle that had done a few hundred miles since being terra cleaned & when he opened it up it was still coked up. You can see from the before and after pics I've posted of a proper carbon clean
Coked up valves have associated problems like rough idle, smoking, poor fuel consumption. whatever remady you use will either fix the problem or not regardless of what the parts look like after.
I had an engine management light come on at 50k miles. (rich at idle) and it was smoking on hard acceleration,j It went in to a main dealer 3 times and an indi 4 times over the next 2 years. No one could put the light out for more than 120 miles. It had countless parts from solenoid valves to charcoal filters /canisters, plugs/MAF and so on and eventually i was talking to guy with and independent Mini dealership and he said bring it in to me and i will hydro clean it, apparently Minis are really bad for this .
Took it in, the light went out, been out for 20k miles now, rolling road showed 381bhp. tick over is such that you might think it had stalled its so smooth and it stopped smoking.
Now people can say its a waste of time and money but it worked for me.