The sound of a bass guitar amp can leak into virtually every track. The result is that you have to turn down the sound of the clean bass track in the final mix leaving you with an inferior bass sound. For live ambient recordings, this isn't a big problem but for other close-mic`d recordings, can ruin an otherwise great live recording. Refer to the Recording Bass Instruments page for more.
Electric guitars will definitely bleed into drums and vocal tracks. When recording live in the studio, either close mic the guitar and keep the volume on the low side or put the guitar amp, and possibly the guitarist, in a seperate space. Another room or closet is best. Another room gives you more choices about where to place mics and room sound but a closet will work in a pinch. A dedicated room also gives you the option of having the guitar and guitarist in the room with the amp so you'll get some acoustic feedback. Fernandez guitars do this electronically (