It's not clear-cut, but an argument can be made that your most aggressive and risky stock choices should be made in a non-retirement account so that you can take the capital loss if it doesn't work out.

I actually take the opposite approach. My view is that my more aggressive and risky choices are things I may want to get out of quicker once my target price is reached - or if I fear a pullback. Since there are no taxes to be paid currently in a retirement account, I try to put the riskier things there since taxes (i.e. think gains) aren't a consideration. In the taxable account, I take more of a buy and hold approach since the long term cap gains benefit is always a consideration.