In tournaments, Larry Harris Tournament rules are often used, which effectively neuters most research by having it come into effect at the END of a turn, rather than the beginning.
You cannot trade in infantry pieces for fighters, even for representational purposes. The original Axis and Allies, if I remember correctly, limited the number of divisions available to what plastic pieces you had - so if you had two plastic subs, you were limited to two groupings of subs (say one stack of five subs and another of three, but you could NOT get a third grouping of subs).
As far as I know, that limitation is not effective in Revised.
For Out of the Box rules, tech is very useful. Long Range Aircraft can give you the added punch to take an enemy territory (key are Western US and Russia), or to have the range to reach a friendly territory to help defend (usually Japan flies fighters to Caucasus). Turn 1 German Sea Lion uses long range aircraft tech to attack London with a transport, six fighters, and a bomber.
Late game, combined bombardment lets you decimate an isolated Japan or Germany.
If you use National Advantages, Heavy Bombers is a cheesy way to win with Allies (US buys bombers that can’t be shot down with Superfortresses NA, and researches Heavy Bomber tech to cripple Germany’s production down by turn 4).