I was reviewing a new environment setting in 10.5 and found that it only supports True or False. The lack of a keyword is (to me) a serious breach of consistency. I took a quick look at the help and see that some boolean environment settings (qualifiedFieldNames) support keywords, some don't (terrainMemoryUsage). The big plus of this is when you are scripting you don't have to guess what True means for parameters where it's not obvious.
I also learned that tool parameters are case-insensitive (cf "ClippingGeometry" for the clip tool) and "true" and "false" can be used for boolean parameters.
Please make life easier for script developers by being consistent. Go ahead and support True and False, but environment variables and parameters should all support keywords.
