Ditto to Andrew and his advice and just don't spray directly on anything questionable, that's all and I've never had a problem cleaning my engine bay. BTW if the quarter car wash has the touch-free rinse, use it, as its always a lower pressure and has chemicals in it that generally leaves a spot free finish. Follow up the hoses and black plastic areas with your favorite tire shine and/or foaming product for that "bling" factor.