If you normally go every 6 weeks and have to stretch it to 8 weeks once or twice in a year - your hair shouldn't fall off your head (although Blaq Kofi did lose a loc or two doing that), what WILL happen is that your retightening session will take longer than you're used to and may cost more - especially if remedial work is required.
Let me explain - your hair normally grows at different rates. At 4 wks this may not be significantly noticeable, at 6-8 weeks your Consultant may notice one section (or more) requires more rotations than another section due to increased growth there... (you can extrapolate)...
The more new growth there is the greater the likelihood of uneven tension being (unintentionally) applied by your Consultant or even yourself (e.g. she may have another client on the way, you may be in a hurry, you may start day dreaming in the middle of a rotation, someone could interrupt you, your arms start to get tired etc etc etc).
It is this uneven tension during the rotation that leads to mr lumpy, mr bumpy and mr ropey living in your head. If you don't mind this - then coolio - but if you paid for Sisterlocks/microlocks because you loved the UNIFORMITY of those many tiny locs - then please pay attention to your retightening.