Explain to me the logic behind marking tanks before a BUFF

Numerous times I’ve seen people suggest it’s better to mark a tank before a buff because requirements will go up. But when a tank is getting nerfed, it’s obviously better to play it as much as you can beforehand then play 1-2 games couple weeks after nerf to jump a little in % as the averages dropped. So using that logic, surely it’s better to not play a tank before a buff then go ham on it as soon as the buff hits to get better games in your better tank while the requirements stay at pre-nerf levels?