There is a small amount of adjustment available from the slop in the stock strut bolts.
But - if that is not enough you can get so called 'crash bolts' from Toyota in 3 sizes (Genuine parts BTW)
They are identified by the number of 'dots' on the head of the bolt.
There are other brands available but I would recommend only using the genuine Toyota ones.
Do you know anyone who can get them for you cheap ha ha?
Anyway you would only need one crash bolt per corner.
If you are really worried about it maybe grab a couple of the middle size and take them to the alignment place when you take your car in, and you may need them for the front as well and they could come in handy later if you go to more extreme settings.
To find out what they look like check out:
http://twosrus.com/catalog/product_info.php?cPath=22_32&products_id=439