From: US Midwest
I have seen several different Axis naval strategies lately. Some players keep their u-boats in large groups to raid, hoping to sink any dispersed destroyers that find them. This works especially well if they have a battleship or two mixed in with their subs. The Allies will be reluctant to send capital ships to hunt these groups, as the heavies are vulnerable to the massed subs.
Other players opt for the dispersal strategy, sending their subs alone to remote parts of the map to raid. The Allied player must weigh whether it is worth it to send a squadron of destroyers all the way across the map to chase one particular sub, who will probably be gone by the time they get there. The Axis player will keep his subs moving and not raid every turn, to make it hard to track any given subs movement.
But the one I have seen most lately is not using the subs to raid at all early in the game, and instead using them to augment the surface fleet. They will often go after the Royal Navy early in the game as part of a Sealion, real or feigned. Then they will attack the Soviet navy in the Baltic, only finally going out to raid after it is sunk. When they do raid, it is usually from the coast of Norway against the northern convoys, probably to keep them handy in case of an Overlord. This strategy does not seem to require the researching of advanced subs until later in the game, if at all.
I am not a fan of using subs tactically instead of strategically, as subs can play a big part in evening-up the Allies economic advantage, which is more important than any tactical advantage gained. Unless you really are going to do a Sealion, then maybe.
Die, by my hand
I creep across the land..
Killing first born man