Has Apple fixed the iPhone 5 sleep/wake button problem?

Apple have not announced any changes in their manufacturing process, nor do they seem to think they have a quality control problem.

Consequently, you do not have any reason to believe that new iPhone 5 units should be more or less reliable than ones they made in the past. If you were happy with their products before, there is no reason to expect you should not continue to be so; if you found their process unreliable before, you have no reason to expect things to be better now.

There's always a chance anyone could get a bad unit, in which case your satisfaction with their customer service policies would then be what matters.

Lots of us think they're making their phones quite well, but if you're dissatisfied, they haven't given any reason for you to think they've changed anything, so maybe you'd be happier with a different product.

I would say the documented issues are anecdotal from the references you have provided. Even if I'm wrong and there is a widespread problem with QC/materials, what problem are you attempting to solve? If your current device works, there's no problem. If your device fails - you take it in for service. Put plainly, no one here has sufficient data to predict how reliable any one phone may be with the variables known to be at play.

This would be like asking if a tire manufacturer has fixed the "flat tire problem" and sharing experiences where someone felt that a tire of a certain brand or price should have lasted over some short tacks and not flatted.

As a consumer, you have the choice to self-insure and risk that you will have to pay for a repair once the manufacturer warranty expires. You can also sign up for extended coverage and protection (and the cost, convenience, and coverage details of those plans vary widely around the globe), so you can control for replacement costs without knowing which factory made your phone, what QC processes may or may not have been in place and whether the people working that shift did their job or did not do their job.