Audio Compressors and Other Signal Processing in 5.1 Mixing
If you use an audio compressor (for reducing the loudest parts of the signal; we're not talking bit-rate reduction here), limiter, gate, or other type of processing for the left and right channels, front or rear, make sure that the compressor or other processing gear is set so that the two channels are linked.
Typically, when channels are linked, one becomes the master and the other the slave. The master channel controls the processing functions, like the threshold where audio compression kicks in and how much is applied. The link ensures that the same kind and amount of processing is applied to both channels simultaneously. This in turn helps ensure that the intended sound localization is maintained throughout an encode/decode cycle (five channels to two, and then back to five) for delivering surround sound to the consumer.
If signal processing is different for the left and right channels (even though the knobs may look like they're set the same) it's possible that the decoded sound may not be positioned properly, and may even appear to wander, depending on the processing and the differences between the two channels.
As always, monitor downmixes and decoded upmixes to make sure the mix heard by the end listeners is the one you want them to hear.