I had the displeasure of helping skin 2 different mature bison at a past job. Both were killed during the summer (August/Early September) and had nice coats. Maybe not as long and shaggy as a winter coat, but they were in great overall condition and came back with good fur.
EDIT: if you plan to ship these to a tannery, you may need to build a sturdy box of your own. The company I worked for had a carpenter on the payroll and he built wood boxes for shipping the 150 or so pound hides. Also Specialty Leather out of Boone, IA, is phenomenal tanning house. They've done a couple hair-on whitetail rugs for me and they are soft and fantastic. They also do a ton of exotics including African plains and dangerous game. They do everything old school, and the tanning drums they use are like 100 years old. Very cool facility and good bunch owns/manages it.