The short answer is they are safe up to about 2", on the ST's rear axle (we're talking about quality spacers, btw). Beyond 2", and you start getting into a gray area where they may begin to add stress to the axle assembly. I'm not sure if there's a consensus on the safety of having them on the front wheels, though. Be aware that some tire stores may not want to mount wheels on your spacers, due to liability concerns. I'd rather do that myself anyway, because it's not a good idea to over-torque the wheel nuts on the spacers, especially if they're aluminum.
Beyond that, the best way to help you is to advise you to use the Search feature. I know people say that all the time, but in this case, it's especially true. This topic has been discussed to great length, and there's no way to consolidate all of it into an answer here.
Click on the Search button at the top of the screen and use "wheel spacers" as your keywords. Select "All posts" and you will find an enormous amount of info.
Good luck.
