So I had planned to write up a big long dissertation here about types of plastic that can be recycled, but conveniently enough, a nice person on The Stranger's Questionland (thanks, Tom) was kind enough to provide a better write-up than I would have (see link below). In Seattle, the number doesn't matter to the recycling company, but other recycling companies in other areas may use them--check with your local recycler to see what their requirements are.
"...the number only tells you what the polymer is but not the extrusion process used which changes the nature of the polymer. For instance, "6" is PS or polystyrene. The most common extrusion is for Styrofoam. Not recyclable. However, those clear clamshells you often see at salad bars etc. are also "6" polytyrene and are reyclable. Those are blow molded and keep much more density than the foam process. Same polymer, different extrusion process and so different recyclabilty.
Bringing your own container for takeout is becoming more and more common though some restaurants hesitate to allow it. "Reduce, Reuse, Recycle" are the "3 Rs" of solid waste management and they are in the order of importance. Reducing the amount of wasteful products is the most desirable as you never have to deal with the consequences of materials or items you never have in the first place. Reusing those items you do need is the next priority and few things are as reusable as tupperware! Failing that, recycling is the next best option.
Some restaurants are uncomfortable with allowing their customers to use their own to-go containers. This is because restaurants spend a lot of time, energy, and money ensuring that the food you get is uncontaminated. They are responsible for making sure their customers do not get sick from what they are selling. As they do not have any control over the customer's container before it is given to them some feel that that is a potential liability for them and would rather you used one of their products which they have more faith in.
Folks do indeed wash out their clamshells. Dirty recycling isn't recycling. It's garbage. Some recycling is so valuable it may be worth washing or cleaning is some way (aluminum) but the economy of scale with recycling does not normally allow for extra steps in the processing."
