Rule of thumb: it's OK to print out one copy of online notes for your own private use without requesting permission (that's why they are posted). So if you and your students will each be printing your own copies, there should be no need to request permission. If you are going to print a stack of copies to hand out, or place them in the library, you should probably request permission. Some notes say what can be done with them.
Added: to answer your specific question, no, it is not customary. I have several online notes which googling shows are often used in courses. I never get requests for permission, except sometimes when the instructor is planning to print copies and hand them out (or sell them at cost) to the class, or when the notes are to be placed in a library.