Building a 10x14 shed this comming week and will be using the Dek-block product. the area that the shed is going into usually has a little mushy soil conditions after is rains for a few days. Not really mud but a little "Mushy". Should I lay some rock under the piers? Or just lay the piers directly on the soil?

Everyone's soil conditions are unique, a person who lives near a swamp will have soil that varies greatly from a person who lives in a drier area and gets some rain. Sometimes just doing what you feel is necessary is the best answer, if you feel you will sleep better at night with gravel under the blocks than perhaps you should add it. Adding gravel under the blocks will aid drainage in any condition, however it is not always necessary. You may also want to see the ground prep area of our support page found here www.deckplans.com/support/faq/buildingsite.html#ground

I had a similar situation with my deck. I dug up the first few inches of mushy dirt from the area and packed the area down, poured out some sand and packed it down some more.
You should try to direct where your drainage is going so you don't get puddling under the deck. I've found that my shed has not shifted at all.
Being able to get your stuff high and dry really makes this product a great shed floor pier.