Yes, the cap should be removed for the float test, but it isn't always necessary. As you know, some will sink with the cap on. I have read that you can leave them soaking for 24 hours to allow the acorns to rehydrate. I gathered about 75 Shumard oak acorns out of a Sam's parking lot this week. About half initially floated. I removed the sinkers and let them soak. I had about another 20 sink in less than 24 hours. Just because they float does not mean they are not viable. The sinkers are in the fridge and I planted the floaters in a flower bed out of the way. If anything comes up, I will transplant it and consider it a bonus.
There is a guide that may be worth your time reading if you are not familiar with it already. It is called Collection and Care of Acorns, A Practical Guide For Seed Collectors And Nursery Managers. The last link I had for it was
http://www.nsl.fs.fed.us/collection and care of acorns.pdf but it doesn't seem to be currently working for me.