If the Axis starts with 6-9 IPC’s worth of additional units(placed before R1), the game is balanced. I’ve played with two basic versions of the bid. Players can bid high to low for the Axis. Example: I bid 9. You bid 8. I bid 7. You don’t think the Axis has a fair shot with only 6, so you let me have the Axis. You don’t necessarily need to bid down one at a time. The other variation involves players writing down their bids, revealing them at the same time, and the lower bidder gets the Axis.
Balancing by tech or NA could be fun, but I haven’t tried this. Obviously, you’d have to be careful about which NA or tech you give. LRA or HB’s for either country would be too powerful, Super Subs would be too weak. German Rockets could be interesting.