All labs, both research and teaching, in our Department have a uniform door/entrance to lab signage that includes contact information as well as some specific hazards (mostly those of interest to our local fire department or other emergency personnel). There is a "biohazard" box to check that asks:
"Biohazards: check if cultures or stocks of infectious pathogens, human blood or other potentially
infectious materials are present in this location."
If the box gets checked, a biohazard sign is also posted.
In addition, per our University's Biosafety Manual:
"In addition to cautions posted at the entrance to the laboratory, appropriate universal warning signs
or symbols shall be placed on all freezers, refrigerators, centrifuges, incubators, waste containers,
etc., where hazardous materials are used or stored."
Any research (or teaching) lab working with BSL 2 organisms, human cell cultures, human blood or other human derived items would get checked (note that all those just listed would also be reviewed by the institutional biosafety committee). We work individually with a PI on if they think their BLS 1 or other items they are working with might meet the above characteristic and need signage.
Since we do use a few select BLS 2 organisms in the teaching lab it has biohazard signage. The ASM document below does not specifically say Biohazard signage needs to be posted for facilities using BSL 1 organisms but it does also contain the following caveat:
"Even though organisms manipulated in a BSL1 laboratory
pose a low level of risk to the community and are unlikely to cause disease in healthy adults, most of the
microorganisms used in the microbiology teaching laboratory are capable of causing an infection given
the appropriate circumstances."
Given that potential risk and given the large variety of microbes used in a college level microbiology class and given the potential for intentional or unintentional culturing of potential infectious unknowns I would still recommend posting a biohazard warning at the entrance of our teaching facility.
The following resources from the American Society for Microbiology provides a lot of guidance on how to manage instructional laboratories: