Some people drift from store to store, but it's my impression most stick with one. The non-Amazon to Amazon shift is a particularly tough one because Amazon uses an entirely different format. Personally, I like having all my books in the same format and easily available on one device.
For that reason, if I were you, I wouldn't waste my time making the reader magnet permafree unless you eventually plan to have other books wide.
Like you, I'm happy with KU, but I do have my reader magnets up wide (including Amazon), and I have one series wide as well. However, that series hasn't prospered, so I can't say it was a great strategy for me. However, there is the argument that it's nice to have a presence outside Amazon, and certainly some people have made it work.