...since nobody has said it yet, the 100-400L particularly does not like "protective" or UV filters.
If you ever move to a 5D3 with firmware 1.2.3, the 100-400L does pretty well with the 1.4x TC III. If in the meantime you want to use a TC with your 70D and the 100-400L, you can mount it on a tripod and use LiveView for focusing. Works fine for more static subjects, or if you have a bird feeder or other place where wildlife or other moving subject may emerge, you can pre-focus on that spot.
Use the lock ring judiciously and unlock it for zooming. Forcing it will eventually cause damage.
Take your time focusing and let the IS settle down before releasing the shutter. Consider the IS as being more for having a steady image for composition than for eliminating shake.
If you get slightly blurred or seemingly a little out of focus shots, first consider whether your shutter speed was high enough and whether you were supporting the lens steadily. IS will compensate for shake up to a point, beyond which you may get motion blur.
Remember that the more distant your subject, the more likely atmospheric conditions such as heat and haze will distort and blur your image. Sometimes you will notice such blurring or distortion in layers when shooting over water, hot pavement, sand, etc. Only at higher altitudes or the rare crystal clear days will you get clear images of distant subjects.
Steady hands and IS or no, you will generally get better images when shooting off a tripod or monopod. At shorter focal lengths motion blur is less obvious, however the longer the focal length, the more obvious any shake will become.
Take your time learning long lens techniques before you think there is anything wrong with your gear.
Use the limiter when shooting beyond 6.5 meters. It will make focusing much easier and more reliable.
You can turn IS off when shooting from a tripod if it is rock solid with no vibrations and you are using a remote or timed release, but if there is any wind or vibrations at less than ideal shutter speeds, leave IS on.
These are just some tips out of experience, YMMV.