WE have dues: $5 per family. We give our membership form to our teachers as well. While it's not mandatory, we really do advocate that teachers become members. This year, we sent an email basically saying, we support our teachers, we hope you will support the PTO. Membership money is income! Also, we provide teacher reimbursement for personal expenditures for classroom materials up to $75. In the past, I've heard that if the teacher who requested reimbursement hadn't paid membership, the $5 was deducted from the total. (not sure how I feel about that but that's what happened in past). We are in a small school of ~400 kids.
Elementary PTO - different strategies at different times, but always had token dues. Usually $3-$5 per family. We found benefit in charging dues, mostly because of the activities around marketing the organization and recruitment. But once membership drive over, everyone was included. Only difference was members got the Class Phone directory for free. (And voting, but honestly that wasn't a big deal.) Teachers dues were waived, but if a teacher was a parent, they still had to complete the form to be counted in the initial contest to see which class could get the highest % of PTO membership.
Middle School PTO - no dues, everyone is a member. As is the nature of middle school, organization much smaller, much less active. Main activity was teacher appreciation and hosting 8th grade dance.
High School PTA - since it's PTA everyone must pay. $10 per individual.
Membership dues/no dues never had an impact on teacher involvement. I suppose charging dues might reduce involvement, but having it free did not increase it. Best results came from having a supportive principal and 2 assigned teacher reps. One attended every meeting and took info back to staff. Teachers were involved by supporting the PTO activities, not meetings. But our activities were always promoted as a partnership with school and supported by the principal.