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There may be other ways to think about this, but for me these are the reasons to play trump during the melding phase:
If you need to start the game with the lead. If you have a lone Ace or if you have cards in every suit you don't want the other person to start the play because you will lose cards that would otherwise take tricks or may not get into the lead at all. So you need the last trick in the melding phase - how big a trump to use is the question: I wouldn't use anything higher than a Jack to save a lone Ace because a higher Trump will probably get a counter in the main game anyway.
If you need to meld before it is too late. If the other person might take the last trick in the melding phase and you don't have an extra 10 of trump then waiting until the last trick to meld your run won't work because you need the trump for the meld so you need to take an earlier trick - and you can probably do it with a low trump if it isn't the last trick. Even if you do have an extra 10 of trump if your opponent has one of the Aces of trump and you are in the lead then they will take your 10 with their Ace - they get a counter and they keep you from getting your run so it is worth the Ace to them. It is worth it to take an earlier trick to meld a run or 100 Aces even if you have to use a low trump to do it.
If you need to meld before it is too late. If you are going to meld 80 kings don't do it after the sixth or seventh trick because you won't have time to use the non-trump kings to take tricks before the phase ends - and if you don't save them in the melding phase you will lose the kings to the opponent's 10s and Aces in the main game. It is worth it to play a nine or Jack of trump to get the Kings on the table early so that you have time to save them by taking tricks with them.
If your opponent needs to meld before it is too late. If your opponent decides to play a 10 of trump near the end of the melding phase and you have an extra Ace of trump then play it - you get a counter and they won't get to meld whatever they thought was worth spending a 10 of trump on. Even if they are leading a low trump or a non-trump Ace it might be worth it to use a nine or Jack of trump to make sure they can't meld.
As always it depends, but if I knew I wasn't going to get a run (or I already melded it) the advice about nine and Jack might extend to King of trump especially if your opponent has both Aces of trump because if they are going to take your King anyway you may as well save it.
Having said all that, using trump in the melding phase is uncommon for me - I only do it in situations where I am confident that the points I am melding or stopping my opponent from melding is larger than the benefit of having that last trump on the last trick of the main game. My father isn't as cautious and uses trump more often so I guess it depends on who you are.