Fashionably late only makes sense if the others can continue without you.
* 2 hours late to happy hour? glad you can make it, get a drink. (But don't expect me to stay another 2 hours)
* An hour late for D&D game? I guess we can start now. grumble-grumble wasted an hour of everyone elses time.
* 15 minutes late to carpool? You better have a good reason.