I usually watch a song's music video once or twice when it's first released just to see what it's like and if it's good I'll end up watching it countless times (
wonderful example here), but usually I'll just play songs whilst browsing the internet or talking on IRC since I've already seen the video once and I'd rather get on with other stuff whilst listening to music. In general I'll grow to like a song much more if there's a great music video for it since it's an instant feeling of "Oh wow, this is awesome", but it's not vital for liking a song by any means.