Keep in mind though that the "fade" issues with Touch ID had nothing to do with the hardware, and were fixed in later software updates.
I don't recall Apple making too many on-the-fly production changes once they release an iPhone model. Even the iPhone 4 with the infamous "antennagate" issue never had a design change with the GSM models (only the CDMA versions released later on had an antenna modification). The iPhone 5 had a midcycle update to add more LTE bands after T-Mobile started selling carrying the iPhone. I don't know of any hardware changes with the 5c or 5s.
Most of the bugs that get fixed are on the software side, and that will happen whether you buy early or buy later.
I bought the 5s at launch, and (at least in the US) it proved easier on launch day than if I had waited a couple of weeks. If you don't want to stand in lines, just wait until Apple's store site goes live at midnight PDT and order it online. Placed my order at 12:06am, and it arrived 5 days later.
In the first hour, most of the 5s configurations were still showing ship dates of "1 to 3 days." By 8am, most configurations had ship dates of "7 to 10 days," with the gold color and T-Mobile unlocked versions showing ship dates of "October." If you waited a week later, most of the configurations showed ship dates of "October" with the most in-demand configurations showing "November" as the ship dates.
So, if you placed the order right when the site goes live, you likely got the phone within a week. If you placed it within the first day, you likely received the phone within two weeks (or longer if you order an in-demand/short supplied configuration). If you placed the order a week after launch, then you might have waited a week for some models and over a month for others.
Going to the Apple Store in the days after launch, you still have to arrive before the store opens and line up. When the store opens, they will let the line up know which configurations they have available on that day, and issue reservation tickets. You won't know which configurations they have available beforehand.
Not sure if the iPhone 6 will play out the same way, but that's how the launch went for the 5s.