Very rarely. I think that it is more useful to use something "in genre" where possible. However, in a situation where a modern similie is more accurate than anything else, I would be prepared to do so.
I frequently use the size of the gaming area and surrounds to provide spacial referents (eg, the cottage is about three times the size of this room, etc...).
As an interesting counter-thought to the original question, if I happened to be describing something that was actually modern (and hitherto, unknown by the PCs) in a fantasy setting, then I would not use any descriptions that would immediately make the item/thing's actual nature obvious (eg, only when the players finally realised that what their characters had found was an assault rifle, would I finally describe it precisely as such).
