I've had a white car before, and new my new white merc would be no different when it came to tar spots. They are easily dealt with and definitely NO CLAYING required.
There are two ways to deal with this.
1. Tar Remover
As already mentioned autoglym do a good one. Spill some onto a clean cloth and and quickly and lightly wipe down an area around 100-200mm square. Leave for just a few seconds and put some more on your cloth then go over the same area again, but this time apply a little pressure. The spots will disappear quickly. You will also see some yellowish marks, not sure what these are, but they take a little more elbow grease, although a little pick with your finger nail will help with these ones. Repeat for affected area.
Now you have a clean panel, problem is, it's covered in what is effectively white spirit, and you do not want to leave that on our paintwork for too long. So, out with the bucket and shampoo, wash down the panel and give a good blast with clean water from a hose.
Finally, I like to add a layer of protection with Autoglym super resin polish. It'll go on real smooth now!
That's the long way....
2. Autoglym Super Resin Polish
Here's the good news, you can skip all that tar remover malarky and go straight to the super resin polish. It has a very mild tar remover in the mix, so apply to a clean cloth and rub directly onto the panel working an area of about 100mm square. Rub lightly at first to remove the easy ones, then a little harder on the stubborn ones.
In this way, you are cutting down on products, removing the spots, AND polishing you car at the same time.
The following weekend apply a good wax and that area behind the wheels will remain easy to clean and tar spot free for much much longer.