It definitely depends on the friend, here. If I'm getting somewhere by public transport (which is pretty often) I'm in that exact situation - I can be early or late - so if there's not a simple solution (eg: a cafe on the way where I can stop for a coffee and read my book for the extra 15 mins) I'll just check to see if it's ok for me to be early. Who knows, they might be coming straight from somewhere else too, and not actually be there.

If it's someone I don't know well, or someone who is likely to be in the last mad throes of getting the place ready, and who I suspect would not want me to be there while they're getting dressed or doing a final clean up, I'll just get there late, and let them know I'm coming a bit late.

Generally speaking, I'd say it's ruder to be late than early, but now with kids and work and everything else, it might not be as simple as people arriving while I'm putting my face on or whatever - I might still have yoghurt in my ear and weetbix on the floor, and an airer full of bras and nappies out, all of which I will be dealing with in those last few minutes If I know people are coming early that's fine (as long as we're going to be here!!) but a heads up is grand. Let me keep my veneer of order in chaos.