It depends on the application -these are not CMOS logic gates!In general unused pins can be left floating, but there may be power savings in some cases if pins are tied hi/lo. <b>What are you doing? </b>
Simple LED flashing/sounds/datacomms PICAXE circuits are not fussy,but READADC stuff can be due to stray signals. A weak but firm pullup/down via 10k Ohms is then usually attached to a pin. Of course you can force pins high or low under software too.