Basic lead strategy in cribbage
If in doubt, lead a 4. This is the highest card on which the opponent cannot immediately make 15. Lower cards are best kept for later.
Remember that ten-cards in cribbage far outnumber any others in the pack. Thus, your opponent is quite likely to have one or more 10s. Consequently, do not lead a 5, or make 21. Naturally enough, 10s are often accompanied by 5s. Beware of making 26.
Conversely, making 11 is generally a good move, providing of course you hold the necessary ten-card to follow up your opponent's.
When to lead a 5
Top cribbage player Bob Milk writes that leading a 5 can be a good cribbage strategy in the following situation:
I am holding a 5 and three 10-value cards and I need one more point to win than I am holding. Example, I am holding 5-10-10-J and any card is cut that does not increase the value of the hand. I am stuck with 8 points. I need 9 points to go out my opponent needs 4 or more points to win (they are in hole 117 or less). You lead the five. In the majority of cards, if you lead the 5, you will get at least 1 go. You may even pair their next lead. There are only a few combinations in which you will not get a go.
If your opponent is playing defensively, lead the 5 as he will be unlikely to pair it. But he may well play a 10-card which you can then pair for 2 points. As with any other game, your choice all depends on the strategy your cribbage opponent has adopted.