Depends... on the weather, year, situation, the Uboot I have.
Recently, I `ve intercepted a small convoy of 8 tankers and 2 large cargo ships in the Caribbean, mid 1942 while navigating IXD2. It was night and weather conditions were good - I stayed on surface and attacked from the starboard, firing a salvo of 4 torpedoes to 4 targets, then I moved a little bit and fired 2 aft torpedos at a merchant - set deep because of the angle and risk of a dud torpedo.
In two nights battle I managed to destroy all convoy except one cargo and both DDs - they were very unexpierienced and ran out of DCs in the first night. It made all the attacks on the next night so damn easier
If the weather is bad enough, I tend to remain surfaced ( or submerge only for short ) and make surface attacks within the convoy at high speed. Of course, such tactics is OK until 1941-2 then it`s just too risky.
In general I adjust the tactis to suit the situation, but there are several things I try to avoid.
I never ever attack in shallow water, that is if there`s less than 75-100m. Preferable areas for daring attacks are 100-200m deep. It`s deep enough to hide and if everything fail there is a slight chance you`ll stay at the bottom and still manage to repair and surface instead of beeing crushed. Of course, it`s just a slight chance.
I avoid attacks in heavy fog and rain - sometimes I use it for getting into a convoy, but on the other hand, there is a risk.
And of course I don`t like areas where patrol planes are often met - if I`m closing a convoy and get spotted by a plane, everything messes up.