This is NOT true.
If one has gone through the process and gotten approval for the PreCheck program (or are Pre-check eligible due to participation in another program, such as NEXUS or Global Entry) AND one's KTN is properly on the reservation, then:
You are eligible to be selected for PreCheck for any given flight. HOWEVER, while it is commonly accepted that most of the time you will/should receive the PreCheck notation on the boarding pass and thus be eligible to use the PreCheck line, it **is not guaranteed** you will be selected for any given flight. [not being selected should be the exception not the rule, but it is certainly a possibility; this is very clear in both the TSA material about the program and when you do your interview they notify you]. Only TSA knows the criteria they use to select the people each flight who get PreCheck.
In addition, as others have noted, only certain airports and airlines participate in the program.
Further, the PreCheck line at any given airport may have specific hours it is open or otherwise be closed for some reason. For example maybe the line doesn't open until 7 am but you are there at 6 needing to clear security.
SW