As far as I know, the default relativeOrder of deployers is Integer.MAX_VALUE, and negative values have no special meaning (except that they're much lower than others).
Setting the value to -1 should be safe, because the Spring deployers do not depend on other deployers (in the same phase).
In any case, please follow https://issues.jboss.org/browse/SNOWDROP-26
A Snowdrop 2.0.0.M2 release is forthcoming and I would like to capture this issue there.