It's really useful to distinguish at least the major groups of soil
invertebrates that can turn up in your plants. Spiders are always
carnivorous and therefore not a concern, aphids are always plant feeders
and do some damage, etc..
In the case of millipedes, most are primarily detritus-feeding and
relatively harmless. But there are a couple of species locally which are
commonly found in greenhouses, which do damage seedlings and other soft
plant growth, often nibbling at the base, making their damage look like
damping off.
Loren Russell, Corvallis, OR