When should you lead trump? The clearest time is when declarer ends up in his second suit. In other words, the opponents bid: 1♠-1NT-2♦-3♦. Dummy will be short in spades and declarer will likely want to ruff spades in dummy. That means you should lead trump.
Also, trump leads against low-level contracts are usually better/safer than attacking leads from honors. However, don't lead trumps from holdings such as Kx, Qxx, Jxx--as it often will cost a trick. Leading trump from Kxx or Axx is usually safe.
Against games/slams, I generally prefer to avoid trump leads--choosing to attack instead. However, if you have all of the opponents' suits well under control, that could point towards a trump lead.