It depends on what you want out of them. You can use them to take weight off the bar in the lower ROM (bands loop under the bar, bands attached above the bar) or you can use them to increase resistance at upper ROM (bands loop over bar, bands attached below bar).
I know this sounds like the same thing but it isnt exactly. For the lower ROM your taking weight off the bar and eventually lifting the weight of the bar alone (at full lockout). For the upper ROM you're using the resistance of the band to "increase the weight (or resistance)" of the bar. Pulling an elastic is harder then letting an elastic pull. (if that even makes sense