I don't have a problem with filtering on either side as long as it is at a sensible speed, takes into account the dangers, and there is an end point that doesn't put you or others in danger. Filtering up the inside is usually in a space that is just too tight, but if the space is there then use it. The top of Bray is a classic place where not to filter. Lanes too tight, no room at the front etc.etc. but the muppets still do it.

Filtering is one of the joys of biking