I learnt it the hard way when supporting small business with accounting software, use to have a client ring at 11pm at night because he was doing his invoicing and things where not working the way he wanted it and it had to have it done that night. He soon stopped after I told him I was going to charge him for call out fees and for waking me up.
I am talking more about if they email you. With phone calls, it is possibly easier to charge a per minute rate. But with emailing, people see that as a way to get free advice and value it less. I do find email is far better for technical support, because you can write instructions and email links, which is very tedious to do over the phone , but it seems to be valued less than a phone call.
But yes I do find that with some clients you do have to set some boundaries, otherwise they will walk all over you.
Oh I no longer check work emails outside of business hours, I found that if you do then people expect you to be on email 24/7.
If they email then it's not urgent and it can wait, if its urgent then they should really call, if i don't answer then leave a voice / text message.
The best thing I ever did was turn my work emails off on my phone outside of standard work hours.
Another good trick is to have a work mobile and personal mobile, then forward calls from the work mobile to your personal phone outside of work hours, this way they never get your personal number and you only need to take 1 phone, just remember to block caller id on the personal phone. Or sign up for a skype phone number and give people that, then just install skype on your phone :)