Generally, the Biorhythm system isn't much to worry about during a run as it's nothing we control and the effects are generally not super spectacular; however, there are a few things to note:

1. Stress seems to affect how easily you're affected by Talk commands of enemies; enemies have a set amount of Stress themselves, but I'm not quite sure how much any of them have. Since Stress is a common bioeffect early on in the game, it's worth noting that having too much can make fights harder.

2. All stats down is a terrible, TERRIBLE thing. Judgement is the stat that affects how fast your "Look" command charges your Guts attacks, and all stats down lowers this stat by what seems to be 3 for the remainder of the battle. The tarots that give this effect are (TO BE EDITED IN LATER ONCE I GET THE NAMES), and are one of the reasons we need so many Survival Manuals to use on Jotaro; it seems to have a 1 turn charge on everything despite that effect, you need 27 Judgement.

3. HP/MP up and down tarots seem to remove or heal for a certain set amount, but do not change the amount of maximum HP/MP characters have.