I would dig around in D17.1. Offhand, I don't recall specific number limits, just pictures of desired, acceptable, and rejectable profiles. Throat dimensions are more applicable to fillets than concavity dimensions. I could be wrong though, so like I said the spec should be consulted. I don't see how a .005" limit could be maintained without filler wire. We do a lot of autogenous fillets here, and most have a concave radius. It is seldom a problem except for the tiny Hastelloy ones. They are crack prone if in the range of .015" when made without filler wire.