Why not add the 5% fee for both conversions HIVE -> HBD and HBD -> HIVE and burn the fee?
Preventing oversupply of HBD is important because HBD is like debt and could cause a death spiral of HIVE price in a bear market as we have seen with SBD/STEEM conversions.
I support the idea of adding the conversion in the same 3.5 day manner in both directions HIVE <-> HBD.
The fee is effectively burned in all the proposed forms of the operation.
While it "might" make sense to add 5% to the normal conversion (HBD -> Hive), it would require more work/testing, and I don't think there's a pressing need for it. And HBD -> Hive conversion is kind of critical to holding of HBD, because traditionally exchanges have had much less liquidity for HBD than for Hive (hopefully we can change that, of course).
Ultimately I think the fee should apply on both sides. With the fee only on the high end, HBD will tend to trade in a range of $1-$1.05, which isn't really desired. Better with the same total fee would be for it to trade in a range of 0.975-1.025, which hopefully would tend toward stabilizing around $1 (vs. $1.025 with the 1-1.05 range).
I agree with your point that this isn't critical so we can ignore it for now in order to simplify implementation. If we end up with stability in a range of $1-1.05 that would be a short term great outcome we could then refine later.