I think that if you go pretty heavy, like 90% for a rep or two, and then drop down to 70-85% for reps, the 70-85% feels easier primarily because you are more efficient, and more "elastic" losing less energy to inter and intra muscular inefficiency. That is not all bad, but you may end up with a pop off the chest with 70-85% that does not mimic the same strength curve as when the 80% is your top load. So you may just throw it through your sticking point all the time, and never get any work in there. When you just do 5 x 3 at 80% the first set is usually the hardest off the chest, and you get more elastic and efficient for 2-3 more sets, and then fatigue starts to set in and the weight gets slower for sets 4 and 5. That way, using a single load you are able to build the whole bench press, (or squat) top to bottom.
If you think about training heavy as basically accomplishing 3 things: 1) cranking up the elastic efficiency of the muscles (Rate of force development), 2) Training optimal motor unit firing frequencies (maximum quasi-isometric strength), 3) Fatiguing motor units, when you do the heavy first, you have accomplished #1 already so speed/ballistic reps are overkill. #2 then is not going to get trained at lighter weights for full range movements because you will be faster than normal, and 3 can be trained, but it actually takes more work to fatigue the motor units because their is less energy lost.
Going heavy first is fine if you follow it up with reps to the point of fatigue, or with movement hybrids that make the lift less elastic, like paused lifts. IMO it reduces the efficiency of the 70-85% zone for 2-3 rep sets though. Also I think that 70-85% for 12-24 reps is the sweet spot for strength gains. But I don't want to make it easier, I want to go through the process of having the first set be a little less efficient, going to more efficiency, and finally getting to mild fatigue on the last 2 sets.
Lastly, when I went to 70-85% I got results. When I started to do a 2-3 rep at 85%+ first, the 70-85 felt lighter immediately, but heavier without the heavy set first. And my progression halted.