This really is a touchy subject... When WinBsp first came out there was an outcry in the gaming community. The reason being that by decompiling Valves maps, you are infact breaching the copyright. It seems that has died down since.
My suggestion is not to modify a map or level, but decompile it to find out just how they managed to get something to work.... problem is that WinBsp has a tendancy to leave out a lot of entities after decompiling, and sometimes you can't tell why something works. But from a brush building view, it is a great tool. Alternatly you can download maps made by a pro and see how they did it, just visit Dave Johnston's old site (http://www.planethalflife.com/davej/). He distributes some of the classic CS maps for free, and in RMF format.
Normally, because HL is 7 years old, anything that might add to enhanced a map has been tried. But if you are just modifying things to help you become a better mapper, I can't see anything wrong with that .... Well, apart from it being Morally and ethically wrong