As the last core troops to be discussed, it's time to talk about our fast cavalry. While no longer the most valuable unit in your army list, like in the previous edition of Warhammer, there are still some things that fast cavalry will do that other units will not (more on that in a later article). In the last book, you usually wanted to bring an equal amount of Spider and Wolf Riders, with Wolf Riders being quicker and the Spiders Riders being able to ignore the movement penalties of forests. Nowadays you rarely have to worry about moving through forests so the Forest Strider rule of the Spiders are no longer as useful. The additional 4" march move of the Wolf Riders along with the cheaper price tend to make them a more attractive choice for regular fast cavalry assignments. In fact, you might even wish to march your Wolf Riders through a forest and hope that you'll lose one of your numbers, as that means there will be too few goblins in the unit to cause trouble (no more animosity tests).
The one time where Spider Riders might be worth more than their wolf breathren is during the Watchtower scenario, where a larger unit of 10 could attack a building and enable you to attack with all ten riders and the spiders, who do fight quite a lot better than wolves. That said, it does make for a fairly expensive fast cavalry unit with a very specific use, who still might not be that difficult for the enemy in the building (your Savage Orcs will clear it out a lot better).
The question with our fast cavalry is not so much whether or not to bring them, but rather how many units you should take. My rule of thumb is to bring one unit of five per 2000 points in your game, but it is quite possible to double that amount to one unit per 1000 points, without spending too much on them. It depends a lot on the rest of your army and your playstyle, but doubling even that amount (which would be common in the previous edition of the game) nowadays tend to mean you're investing too many points into units with limited use.
Either way, the important thing when using fast cavalry is to be patient. The longer you can keep them alive in a battle (without investing too much in doing so) the more useful they will become. While it might certainly be a great idea to vanguard early and try hard to charge into a war machine, if all you do with them early is to sacrifice them as a speedbump for a mighty foe it might be better to just keep them in your deployment zone in the early rounds. If you can consistenty keep your fast cavalry alive until rounds 4-6, you'll be surprised at the opportunities that present themselves where such a fast and mobile unit will save you the game.
Unit size: No matter how many White Dwarf articles and fancy pictures you end up looking at, the best thing about our fast cavalry is that they're cheap, and we need to keep them so. Bringing more than the minimum size of 5 will disappoint you, since goblins are absolutely terrible in fights, no matter which way you look at it. House-invading Spider units of 10, or medium cavalry of 15 Wolf Riders might be fun to try out, but they probably won't win you many games.
Equipment: As keeping your unit cheap is the most important thing, you'll want to equip your fast cavalry as little as possible. Spider Riders come with mandatory shields and spears, which are of dubious use, while the Wolf Riders have to buy all their stuff. If you have the five points to spare, it could be fun to give them short bows and have them plink away at something. I once took the last wound of a Shaggoth about to flank charge my Orcs with five arrows from my Wolf Riders. Since then, I always make sure I have those 5 points to spare.
Command options: Your fast cavalry units should never be fighting anything where challenges will be involved, so bringing a Boss is out of the question. Standard Bearers might be of some use for vitality-based scenarios where lots of banners are good, but adds to the point cost for little benefit. However, since fast cavalry who elect to flee as a charge reaction and manage to rally in the following round get to move and march as normal that same round, it is well worth bringing a Musician to help out with those rally tests.