Currently, the only guaranteed support is for layers greater than or equal to 0. While there is nothing stopping negative layers, there is nothing guaranteeing they work (they have not even been tested). Should explicit support be given, or should negative layers be rejected? Either way, half-support ("oh hey, guess they work right now") should not be the policy.
It is worth noting that changing layers (and thus adding a layer underneath all of the others) is simply a process of setting the layer for each model needing bumped. No heavy recalculation needs done based on the implementation of the internal renderer. Currently, nothing guarantees that behavior going forward, but there are plans to make that behavior guaranteed.
Currently, the only guaranteed support is for layers greater than or equal to 0. While there is nothing stopping negative layers, there is nothing guaranteeing they work (they have not even been tested). Should explicit support be given, or should negative layers be rejected? Either way, half-support ("oh hey, guess they work right now") should not be the policy.
It is worth noting that changing layers (and thus adding a layer underneath all of the others) is simply a process of setting the layer for each model needing bumped. No heavy recalculation needs done based on the implementation of the internal renderer. Currently, nothing guarantees that behavior going forward, but there are plans to make that behavior guaranteed.