as far as the standard eye exam goes presumably most of it is pretty objective, you can either read some letters or you can't... a particular lens looks better, worse or the same etc..
can't see why it would be a big issue unless you've got some problem with your eyes and want a subjective 'expert' opinion from the optician, then it probably is more like GPs, dentists etc.. where they do seem to vary considerably in quality
I do have problems now, but have been using an independent optician since my teenage years, long before the problems.
I view it as a kind of insurance policy, on a critical asset that can't just be replaced. A really good optician might well catch problems developing before they get too serious, allowing a better chance of remedial or preventative measures. Given that, like teeth, you only get one set, and once ruined, they're ruined for good, the best you can afford is a sensible investment.
Putting that another way, I'd rather pay for quality than even risk a mediocre optician missing something. The trick is telling the one from the other, and fee level isn't always a good yardstick. I've known expensive private doctors rely on age, reputation and PR, and fall well behind younger, more up-to-date practitioners.
The problem with many chains is that it is a bit of a crapshoot, and having found a good one, that individual may not be there a year or two later. At least with a standalone practice, they're likely to stay put, once established.