While I said there were no real general guidelines, there is one notable exception. That would be a hand where you in sit in first seat, hold three, and the dealer has named that suit trump. Here is where a trump lead would be the best play. Why? Because if you hold three trump chances are the maker also holds three. A lead of your middle trump works best. It would not be wise to lead your top trump as this may allow the dealer to control the remaining trump. Keep in mind the maker will need to use a trump to take an off-suit ace. You can use this fact to draw out another of his trump. This may leave you in a position to control the balance of the hand. The vast majority of times a trump lead will result in the bidding team being euchred.
The dealer sits in the East position and turns-up the jack of spades. Everyone passes and the dealer picks-up the jack, discarding a junk 9 of clubs.
South is hoping the dealer (E) would turn down the jack, but that was not to be. So instead, he thinks about trying for a euchre. A trump lead may just do the trick. Let's start things off with the king of trump. East plays the 10, North throws off a 10 of diamonds and the dealer/maker(E) takes the trick with the ace. They took their first trick with three trump played.
Now the maker (E) leads the king of diamonds, He's trying to set his hand up for an end play. South avoids the trap and wisely rids his hand of a useless queen of hearts. West has to follow suit and North takes the trick with the ace. Now both teams have a trick
Next North leads the ace of hearts. The maker (E) has to follow suit. Again South throws off. Also holding a heart, West has no choice but to follow suit. The ace is good and gives North/South their second trick. Now, no matter what the maker does, he's euchred.
Now look at what may have happened with a different first lead. Let's say it was the king of clubs. East would have taken it, and maybe lead back the 10 of clubs. If the maker had trumped in they would have made a point. OK, how 'bout leading the queen of hearts. Yes, this too would have resulted in a euchre. Nevertheless, until the hand is played out, no one would know what cards are where. The trump lead gives you the best chance of the euchre.