As the format increases, depth of field decreases but diffraction also decreases at any given f-stop. F64 with an 8x10 is equivalent to f8 with 35mm, so f64 is a moderate aperture for 8x10 and does not particularly need to be avoided. This is where the f64 group gets its name, but using 8x10 cameras with a moderate f-stop setting.

F64 is too small an aperture to recommend for most 4x5 work. Maybe your school was taking the f64 principle common for 8x10 and applying it to 4x5. F32 is the equivalent aperture for 4x5 (similar to f 8 for 35mm. I like to stay within the rage of f16-f45 for 4x5 work and f32- f90 for 8x10.