Not sure exactly what you mean by bulk, ie if buying for individual use or resale, because yes freshness is key with sencha and matcha, due to the fact that the tea is finely cut so there is a lot of surface area. I wouldn't buy more than you can consume in a month or so once opened.
This is my favorite Sencha I have come across:
Ito En Chiran Kanayamidori, from Kagoshima
And this organic matcha is awesome, better than the conventionally-grown matcha's I've tried:
O-Cha Organic Kaoru Matcha
They say that the best tasting tea is not organic, because organic certification costs quite a bit, and many growers prefer to spend their money purely on making the best tasting tea.
I have heard that Japanese tea is generally safer because they don't use water-soluble pesticides, and as a more developed country their agriculture is more closely regulated, whereas who really knows what goes on in China.... Still, I love Chinese tea as well...