On this representation negative is always undesirable and more positive is always better. It isn't a "normal" RSSI number (which I find annoying, but it is what it is). I think what is being shown is really a Signal to Noise Ratio (SNR), not RSSI.
On this number one would like it to always be positive, but that said I have devices in the negatives that work perfectly fine - but it all depends on how chatty they are and what the reason is for the low RSSI - interference, antenna alignment, etc.
I'll just throw out that I have nothing higher than 26 dB - I forget what the max is on zwave (and I think it is different between 500 and 700 anyway), but it is likely somewhere in the 30 dB range I would guess based on the protocol, radio, and antenna type.
Typically below ~ -5 is where I start to get a bit concerned. But even then, one of my Ring Extenders insists on routing directly to the hub, and has a -9 RSSI and is working fine, so 
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