Further, if you to witn one of the fast ones, i.e. one that has an internal clock faster than, say, 10 MHz, you can start to worry about crosstalk between signals due to the very fast rise times and the typically 5 pF or more capacitance between adjacent terminals on the "breadboard." The CMOS inputs and outputs are very high and low impedance, respectively, so the high-impedance input "hears" the energy in the low-impedance output. There's not much you can do about that.

Down around 1 MHz, as in the classic 12-clocker, it's not so bad.

Reducing the clock speed does NOT help, as richard states "the very fast rise times" is the issue and have nothing to do with the clock speed.

One issue that may be missed reading Richards post is that a devboard is designed to work at the speed of the chip, so, till you start hanging stuff directly on the port pins, this is not an issue.

Erik