Lately I have been tanning most of what I catch, but I grew up using pine boards. I do have wire stretchers which I have used, but there is a risk using wire when selling fur [I think]. Others that are currently selling may be able to comment here, but I think fur grading is based on specific width & length measurements in addition to quality. Using wood stretchers, you can best ensure that your dried fur conforms to those specs and get the max value. With wire you have give and take going on between width & length depending on how firm you pull down, so you might end up a little off the standards and possibly loose some on price.
I will probably sell some otter raw next season and my current intent is to use pine boards exclusively for them.