Thanks for the replies folks. As JustMarie and EricEyster made opposing arguments, I guess the most important point is to be consistent!
Jeff, I don't see it as much of a pain to maintain a IsCurrent flag; I agree that it could be seen as superfluous, but I'm just following Kimball best practice which states:
...the current-flag provides a rapid way to isolate exactly the set of dimension members that is in effect at the moment of the query.
So it's for ease of querying more than anything else. Thanls for the tip about the EndDate value though - I think I can afford to lose most of the year 9999 :-). (Although thinking in that vein lead to the Y2K problem didn't it? ;-))