If you are planning to buy a mac then you are planning to target a mac audience, which means you should look at what most mac owners are using, and it isn't integrated graphics.
You're assuming that 'most Mac owners' constitute the casual audience -- do we actually know this? Do you not think it is conceivable that Mac owners with better than integrated gfx might, by virtue of having those cards, actually prefer to spend their money on the more mainstream games that they can run? We can't really call this without some reliable stats. It's just my opinion but I think you'll be much happier working on a mac with a graphics card
I want to agree, because if you shouldn't skimp on chairs then getting the nicest dev machine you can afford clearly follows. Personally, though, I would not be comfortable splashing out on the more expensive models without AppleCare, so the leap in price for such machines ends up quite significant -- unless the stats mentioned above backed up this decision, I would wonder if the expense would add all that much value.