From what I recall from my tents, most companies suggest wiping down with a mild soap. I would think putting your tent in a washer might be too hard on the seams, stitching, zippers, coatings, etc. I did a quick internet search and below is what Sierra Designs (first company that came up) says on their tents:
"Proper cleaning and storage of a tent will prolong tent life. Your tent should be cleaned of all mud, loose dirt and debris when you return from your camping trip. Shake out any loose dirt, and wipe the floor and fly clean with a sponge and water.
Make sure your tent is completely dry before you pack it away. A tent that is packed away while damp will mildew. Storing your tent loosely in a large stuff sack or box may help prevent the formation of mildew, especially in humid climates.
For a more thorough cleaning, hand wash your tent in a mild, non detergent soap and water solution. Down Soap works well. Do not soak your tent. Rinse thoroughly and allow to air dry out of direct sunlight. Never machine wash or tumble dry your tent."
If you know the brand of your tent, maybe they have their care instructions on-line as well.
Good luck.
Horniac