The importance of image stabilization

Using a panny body with oly lens, on shorter focal lengths you shouldn't have much of a problem. from 40mm and on I think it becomes a challenge if you're not in outdoors sunlight. I think the rule of thumb is 2x effective focal length as a shutter speed (so 150mm is 300mm equivalent, so minimum shutter speed should be 1/600s for handheld shots).

I've been using a GX1 and so I haven't been able to use Oly lenses much... the 45mm/1.8 is somewhat usable because of the bigger aperture but it too struggles indoors sometimes. I think I have shaky hands too.

I just got the OMD and the 5-axis IBIS is amazing. I can handhold shots at 200mm indoors and get sharp results.

So conclusion: Yes I think IS is important. If you're going to go with a panny body, look into the panny 45-150 or 45-200 with OIS. I'd recommend the OMD if budget permits.