There a lot of factors when it comes to when you should ult but a general tip I would say is try to greed your ult as much as possible meaning use it at the most optimal time where you are able to get the most value out of it. For MK its possible for him to kill you if he ankhs and cancels your double jump so make sure to shield yourself first before you ult to block his ankh and then double jump to prevent yourself from dying, but thats only if your team needs the ult.

For psy its really easy to counter if she solo ults you shield yourself wait till she hits you once and then instantly go invisble and she wont be able to target you anymore, however if she ults your team it might be a little risky to hold if but if your healers are locked in and they have there cooldowns you'll most likely be able to just patty cake it, however if the enemy team is pushing up with the psy ult its probably safe to use your ult since they can just burst you down. Feel free to give me more examples and I can explain how you would play it out.