Is it okay for a potential post doc to send an email to professor on the weekend?
Email is a form of asynchronous communication: it doesn't matter when mail is sent, it can be read whenever pleases the recipient.

High-ranking professionals should not sacrifice their operational efficiency to avoid the possibility of lower-ranking professionals feeling stressed, overwhelmed, etc. by out-of-hours emails. High-ranking professionals should make lower-ranking professionals aware of work-life balance. Managers and subordinates should discuss expectations. 

Technology can help. Email should never raise notifications. (They're too numerous and interfere with flows.)  Messaging apps can be segregated: Some for work, others for home. (Signal at work, WhatsApp at home, for instance.) Work apps should only raise notifications during set hours. 

Only individuals can establish their own, "best" work-life balance.