Bypass capacitors needed in low-frequency digital logic systems?
The relevant factor is the rise/fall times, not the clock rate. Here are two relevant papers.
Conventionally, designers stick to 100nF cap per IC. Bear in mind that they serve multiple purposes: signal integrity, power supply noise, internal IC operation, radiated EMI, susceptability to EMI. Using an SMT 0805 or smaller (smaller is better) shouldn't take too much board space.