The answer is: It matters. You can put monster truck tires on your car if you want, it's all a question of what other work you want to do to the car. I also have to make a bunch of assumptions to answer your question because you didn't give much info on what your goals are.
If your goal is to maximize tire width without touching your suspension or modifying your fenders, the answer to your question is 235 width. Generally 17x8" +48 235/40R17 is a good choice.
If you want wide wheels, not wide tires, the answer is different. Also, adding in fender and suspension modification will change the answer as well.
Stock offset is ideal for stock and stock-like suspensions, regardless of tire and wheel width. That said, you may need to reduce offset to fit wider setups. Wider tires are good for many reasons but there will be some determent to the handling for reducing the offset. Staying close to stock offset will help minimize that.
2015 STi: Still Stock
04 STi, 04 WRX: SOLD