From what I've seen it all depends on who management wants to pick on. A friend friend mine used to be a minute or two late quite infrequently and they pulled him into the office once day to discuss this "problem". But yet we have drivers who show up daily 15-30 minutes later than their scheduled start time and nary a word is said.
The only "rule" I know of start times is that if you are late for you scheduled start time then you are not guaranteed your 8 hours of pay. If your start time is 8:30 and you punch in on time and then punch out at 17:00, with an hour lunch, then you are still guaranteed 8 hours of pay, though you were only on the clock 7.5 hours. BUT, if your start time is 8:30 and you punch in at 8:32 and still punch out at 17:00, with an hour lunch, then you will be paid straight time and only be paid 7:28. You could change your start time in your DIAD but then you could possibly be subject to falsifying documents and possible termination.