For ab initio instruction the student is a blank sheet. You are their only frame of reference and they will emulate you. Everything you do will at its foundation be an example of good airmanship or bad airmanship. It is absolutely vital that you don't cut corners, rush, or let your standards slack off because that bad example will infect your student.
If you are in a big rush to do the walk around, have your stuff in a disorganized mess in the aircraft rush/disregard the checklists, taxi too fast, didn't make a point of watching your prop wash, blocked a taxiway because you did your run up in a poor spot, have lots of UMMs and AHHs in your radio calls because you started talking before the brain got engaged or stepped on somebody transmitting because you did not listen out, etc etc then you are contributing to the next generation of crappy PPL's
If you say to your student that cruise altitude is 2000 feet yet let the aircraft stabilize at 2100 feet and don't immediately correct then you are saying to the student + or - 100 feet is OK. If you brief final approach speed is 65 and fly the approach at 70 then you are saying speed control + or - 5 knots is acceptable etc etc then then you are contributing to the next generation of crappy PPL's.
You are probably thinking that "Oh I will never do those things", but the test will be when you are on your 6 th student of the day and have already missed lunch. Or you haven't flown in a week and the weather isn't really quite good enough for a proper lesson.
Good instruction is a choice. Be hard on yourself and demanding of your students.