I like a representative sample, leaf tissue from an area, zone or field.  It makes more sense to match up with the way you soil sample.  I don't want much variation in size, color or stand but that is your choice.  Most fields need a soil sample pulled where the tissue was pulled to get a better look at how we are doing.

Yesterday I pulled a 10 acre field by itself, probably 20 handfuls of young wheat in the sample bag.  If you don't have lab bags you can use a small brown grocery bag(which are becoming rare!).  I pulled a 50 in 2 samples, bottom ground and terrace ground.  They will match up to my soil test better.

You can pull one per acre or enough to fill the sample bag in any form or fashion you want and find out what the plant is taking up.  Sometimes the nutrient is there and the plant can't get it like Magnesium, it just doesn't flow well in cold weather.  P could be tied up.  Wheat is a heavy user of NPK CaMgS B Zn and is a good foliar feeder plant though I like dry broadcast as well as anything.  I am not feeding just this crop but every crop thereafter.

https://www.google.com/#q=how+to+tissue+sample+wheat

https://www.google.com/#q=how+to+tissue+sample+agphd

Hope this helps you a little bit.

Ed Winkle