There's the issue of the SOC indicator being confused when you start up, and there's the issue of damage to the battery. I think that the NiMH IMA pack will self discharge and confuse the SOC indicator, but that won't damage anything. But the lead-acid 12 V battery will get damaged more if it is allowed to dwindle. When I leave my Insight for more than a week, I like to put it on a 12 V battery maintainer.
I do think that the IMA pack self-discharge goes faster at high temperatures. I once had the experience of leaving the car for about 3 days in 90+ temperatures on a wide-open asphalt parking lot. The temperatures inside the car were surely well above 100, perhaps 120. The pack was nearly full before the 3 days, and I had a re-cal almost immediately when I left the parking lot.
Here's a good reference that shows on the bottom of p. 10 the
self discharge vs. time and temp:
Panasonic NiMH battery manual
At 20 C, you can leave them sit for 4 weeks and lose less then 20% of the
charge. At 45 C you lose 40% of the charge. But in neither case are
the batteries damaged. In fact, I've seen it recommended to store NiMH at about 40% charge. That's in sharp contrast to lead acid which degrades rapidly if it sits at anything below full charge.