Been fighting with my Millermatic 185....

To answer your question about using a tip one size bigger than the wire.If a spatter ball hits the tip at the wire there is more room around it to keep feeding wire same as if the tip gets gummed up.Catch 22 though,the bigger clearance also allows a bigger chance of getting stuff into the tip further down.Example if a spatter ball hit the tip hole it can form into the hole further making it hard to clean out and some times near impossible.It also let the spatter ball get hot while welding and makes the wire drag.I prefer to use the right tip for the wire but either way is fine just different strokes for different folks.