For big AAA developers, sure, it would make sense to make a MAC version at the same time as other versions to some degree, since you're already likely developing for some platform that has to use a variant of OpenGL anyway, assuming you want to make the effort to go after Mac and Linux systems to begin with anyway.
But for indie devs? I'm going to have to say that making a Mac version while making the PC version may not make any sense for them. It's a tiny market and they're often limited both in cash flow and man power. Developing the same game simultaneously for multiple systems simply isn't that feasible when the primary system with the largest market share is so different from the others. You're basically doubling the workload in a lot of development areas and for what? An extra 4% of the market? Simply not worth it as an up front expense for a lot of devs. Now once they release on PC and start making some money it may be worth it, and they'll certainly have more ability to do it, but I wouldn't say doing it before is the best idea.
Moreover, if an indie dev was going to go back and release on another platform after Windows PC's, Linux systems would be a better choice anyway. The market size is somewhat comparable to Mac OSX, but Linux users have repeatedly demonstrated that as a percentage of their total user base they are just plain willing to spend more. They spend more on indie bundles than the other two major OS groups for example, and there's definitely a desire amongst Linux gamers to get more games that support Linux natively, whereas I doubt most Mac gamers really give a shit after years of Macs getting diddly for games.