When I rebuilt my car's cylinder head this was the case on mine. The pintle caps on the injectors were very deteriorated, but all except one still sprayed 'ok'. The carbon buildup on the pistons was under 2mm thick plus the head had buildup inside the chambers so the overall volume of each chamber was reduced by perhaps 1.0-1.5 cubic cm (1/2 a cubic inch).

I was tempted to leave it to keep the compression up a bit, (we have better gas than when they made these cars) but I ended up secumbing to the temptation to match those precision honed bores (exhibiting no wear) with smooth, shiny piston tops...

I bet it makes no difference at all... GP