It depends on a lot of factors--particularly the space you are singing in. However, the placement of not only sections but also individual voices within the ensemble can have a major impact on the overall sound. Follow your instincts, experiment and do what works best.

Volume can be an important part of blend. My altos are weaker than my sopranos. Some time ago, they requested to not sit behind the sopranos since they were overpowered by them. They now sit in front of the basses.

Every placement policy depends on the singers in any given choir. Experiments with what sounds best in one's acoustical environment are paramount. In some instances, a weaker section may be improved by having it in front of a stronger one. This may result in them blending with what they hear behind them - as well as singing out more if needed. For choirs with chronic vocal production problems time spent on voice paedagogy will be well spent and yield better singers and a better choir. Where numbers and talent are not an issue some have placed their choristers in quartets. This produces a very smooth and well blended sound in which all blend with all other pitch registers - it also ensures that all know their parts well and are not dependent on their neighbours. At any rate, no placement should be thought of as permanent. New members, changing voice quality of old ones, this repertory vs that repertory, and other factors may make moving singers around a positive musical policy for long or short term.

It can also have a major impact on the sound of the choir if one section is placed physically higher; e.g. recently for a concert we placed half of the basses and tenors one step higher behind the other half instead of in one long line behind the altos, which improved hearing the other parts and made the bass and tenor part more prominent.