99% of what I do is analog audio so depending on what you work on will change slightly. Personally, I use sine waves to check for loss of signal, distortion, checking for and trying to excite unwanted oscillation, signs of clipping and overdriving gain stages etc. I'll inject square waves primarily to check filters and look for ringing on power supplies, but they can also be useful for testing and analyzing digital switching and logic gates. Triangles, I pretty much never use it. As I said, depending on your interests your mileage may vary.