I have never been to a WRC event, but I spectate at regional and national rallies.
First, get a spectator guide, these will show you the locations and times of everything worth seeing. If there is a super special, try to attend it. You may need to pay, but these are set up for the spectators to see as much of the cars as possible and are worth it. I like seeing the Parc Expose in the morning, it is a great place to see all the cars and the drivers/codrivers and get amped up for the day. The service is another place that is fun to see and worth a trip.
The stages themselves can be hit or miss. Where I live is thick forest so there can be lots of standing on a corner half a day to see a car for 10 seconds as it goes by. If you can find a high point with a clear vantage of the stage that is great, however here in the sue happy US the spectators are tightly controlled with spectator areas being picked for safety over good views usually. I have tried both getting to several stages, one in the AM and one in the PM, as well as posting up all day on an "out and back" stage (one that is run twice). Not really sure which I like more. I think the real hard cores are out on stage all day with a barbecue and a ton of beer, but I have usually tried to see as much as possible during the day myself.