As a general rule it is best to lead low in order to avoid taking unexpected tricks. However there are lots of exceptions to this rule.

It is usually not advantageous to have the lead. The ideal position is to play last. A lead of trump is good if it is done with a purpose in mind. A small trump may be led to force out other trumps. A high trump may be led to win a trick or to force out Wizards or higher trumps.

The particular lead depends on whether the player is attempting to set up a trick or trying to unload a card. This in turn is often determined by whether the number of tricks called for by the table is over or under the number of tricks available.

Its best to keep your options open and your opponents guessing so Wizards and Jesters should generally not be led. It is usually better to save W&J for crisis situations which often arise during the end game. Of course there are situations where the lead of a J or W is the best play.

Often a hand will contain cards of dubious value that may or may not win a trick. It is usually wise to find out early in a round whether such cards will win or lose so that adjustments can be made if necessary. For example holding a singleton Jack of Spades, it may be prudent to lead the jack early in the game rather than be surprised later when it wins a trick. If the Jack lead does win it is usually possible to fluff-off one of your expected winners.