Memory interleaving is accomplished by installing two DIMMs in corresponding slots. That is, one DIMM in A1, and another DIMM in B1 will set the machine up to use memory interleaving.
If you have an odd number of DIMMs, the matched pairs will run the memory interleaved. The odd DIMM will then run non-interleaved. For the interleaving to be most effective, the DIMMs must be the same size and speed, (usually, should be of the same manufacturer, but not necessary). In reference to the memory addressing, the A1/B1 will be the lower addresses, going up to the A6/B6 being the highest address.
In relation to performance, it really does not matter where the DIMMs are placed. The software is intelligent enough to figure out which banks are being used, and is able to "stitch" the memory together as required.
Note: Memory interleaving is only available in the Power Macintosh 7500, 7600, 8500, and 9500 series computers. Other Power Macintosh computers do not support interleaving and no performance gain will be realized by populating memory in pairs.