I always add external resistors to pullup switches for a number of reasons.

First I get a faster rise time when the switch is released. This is even more important when the switches are used in a matrix.

Second I want more contact current than that offered by the weak pullups on the 8052 type chips.

Third the resistor lowers the impedance of the connection thus making the mode much less sensitive to false triggering and noise.

Michael Karas