Math skills needed for case interviews

Hi all,

I have been practicing math for case interviews for potential interviews next summer. I have been practicing through Cheng's site and other things and have got these skills down fairly reasonably.

I recently came across some resources that says simply doing the calculations is not good enough and that calculating things that long way isn't going to cut it. All of these sites say the same thing, but they also happen to be selling math products that promise to remedy these problems.

I am wondering how true is that? If you take a bit extra time to calculate something like $32 x $25,000 the long way vs mentally breaking it down and doing is quicker, how bad does that really look? Do firms say, "Well this person calculated their numbers the long way, they're not Firm material"?

Thanks in advance