You can never know how any open available proxy reacts.
Open proxies are either a) hacked computers or b) wrong configured proxies.

In case a) the longtime stability is usualy low, the owner gets a bandwidth bill and stops the proxy b) once the admin finds the error the proxy might suddenly be non anonymous or locked

I'd test them every day you plan to use them to be sure you don't send your real IP and possibly ruin all the work you invested