well, bit depth describes "the number of bits of information recorded for each sample" (wikipedia)
A higher bit depth in the real world allows you to record an audio signal at lower levels (thus avoiding possible clipping) while maintaining an accurate representation of the original audio signal.
Basically, in a Logic project no need to worry if you're using 16 bit and 24 bit files together.
The crunch comes when you get to your final bounce. Now you need to consider the sample rate and the bit depth depending on your delivery medium (i.e. will the music be on an Audio CD, for film, for Audio DVD, mp3 etc etc).
For example, an Audio CD is a 16bit and 44.1khz compatible device.
So, what to do if your Logic project is at 48000hz and contains 24bit audio?
choose the correct sample rate (44.1khz) &
dither... from 24bit to 16bit (this can be found in the Bounce window) and dithering essentially adds a little noise to your final mix.
Remember though, only dither or change sample rates on your final, final bounce otherwise if end up dithering twice you'll compromise the quality of the audio.