I suppose you’re thinking of WWII. It seems to me that practice varied for different air forces at different times in different theaters.
In New Guinea in 1943, US A-20s and B-25s raided Japanese airbases at tree-top altitude to avoid flak, coming in so low that they developed a new type of munition, the ‘para-frag bomb’, a fragmentation bomb on a parachute, so that the raider could get clear of the target before the bomb load went off.
On the other hand, I was just reading Danny Parker To Win The Winter Sky, a detailed account of the tactical airwar in northwest Europe 1944-5, with special focus on the Battle of the Bulge. Without pulling the book to refresh my memory…Parker recounts that one of the senior commanders in US 9th Air Force had the fixed idea that his medium bombers should operate like fighter bombers, coming in low and fast to hit their targets with pinpoint accuracy. A couple of times he sent medium bombardment groups on low-level missions, and they were shot to pieces by German light flak. After that, the mediums bombed from altitudes above the German light flak.
I don’t know why the low-and-fast tactic worked against the Japanese in New Guinea but failed against the Germans in 1944.
I expect to get an accurate picture of medium bomber tactics, you’ll need to read up on the particular campaign that interests you.
You'll shoot your eye out, kid!