I can say that I used to be an absolute fanatic about the fact that I felt Germany should always hit Karelia round 1. (note, this is before bidding.)
Here is why I changed my mind:
1) Germany has juicier targets with their Luftwaffe round 1. For instance, hitting SZ 2, 9 and 12 effectively clears the British from the Atlantic buying Germany a number of rounds (at least 3) before having to worry about a serious invasion from the west.
2) Karelia is only 7 IPC (2 territory, 5 national objective.)
3) You do not kill any Russian armor or fighters/bombers. (Because the only one on the board is in Moscow.) So you are only taking out 19 IPC of units, none of which terribly valuable, and risking fighters (and bomber?) in the attack. Not a great trade.
4) You can still hit Ukriane, East Poland and Baltic States heavily enough not to lose them in a counter attack if you do not commit heavily to Karelia.
For all these reasons, and more unstated ones, I currently do not favor an assault on Karelia on Germany 1, regardless of bid for the allies.
In a nut shell, here is why I used to support it:
A) 19 IPC is over half the Russian income and you are taking a forward base.
B) If you plan the attack right, you can end up with a couple armor to defend effectively denying the Russians the Industrial Complex and sealing a 5 IPC NO for Germany for potentialy the entire game.
C) If you are truly daring, as I was in my youth oh, about 15 months ago, you can take fighters as casualties had have a REALLY strong ground force in Karelia on Germany 2, perhaps strong enough to stack Belarus heavily and force the Russians to stack Moscow on R3.