Been riding my 2.1 Trek alot lately and have a question thats been nagging at me big time. Gears

If I'm in the small chainring up front (50/34)compact?? can I use all 9 gears on the back and not have it rub the front derailuer? I know, total newb question. The reason I ask is that I get chain rub on the FD when I use all gears on the back.

Are there guidelines to be used when say for example in the small front chain ring or the large up front only use certain ones in the back?

On flat rides..Should I favor the small or large up front? Does it matter?

Thanks a million in advance