I think you are right, and it might not make any difference since you already have OEM roller lifters. However, many folks also make other changes at the same time, like switching to 1.6 ratio rockers as part of an upgrade, and that might make a difference in pushrod length.
I also had to buy a pushrod length checking tool and a better valve spring compressor to remove and replace my new dual valve springs when I checked the length, which were costs I not figured in ahead of time for the conversion.
I read several articles about roller rockers, and finally decided to buy Summit brand stainless steel full roller rockers, since they were a little cheaper than aluminum. However, the price point seems to change frequently. I also lubricated and checked each rocker for good movement, and found one that I had to pull off the roller snap ring and reseat it so that it moved well.
Bruce