In your case, you don't really need a relay. Relays come into play when you have a device (horn, starter, lights) that run off a switch (i.e., toggle switch, starter switch, etc) that require a large current draw. Because of this draw, you need to have a supporting infrastructure to support that large draw so that it doesn't melt your wires, switches, etc.
Also, because large current draw equals high heat, wiring and switches that aren't designed to handle high heat will have a short life span as they will "die" sooner.
If you design a system by which all the wiring from battery/fuse box to switch, to device are strong enough to handle the load, then you're driving up the cost of the higher quality materials.
Enter the relay.
Basically, a relay is nothing more than a switch (but NOT like a traditional toggle switch) that handles the current flow between your battery/fuse box and device. You then wire a toggle switch to operate the relay. In a nutshell, you hit your toggle switch to turn the relay on or off, which in turn tells the relay to turn your device on or off. The load between a toggle switch and relay is very low, especially compared to the load between the relay and the device. So your toggle switch will last much longer, you won't melt it by running high current through it, and your circuit is much safer overall (melting/shorting wires will produce high heat which can produce fire). Also, you can purchase a 12v relay for about $10 or so, so it becomes more economical for high load devices.
Since you didn't put a switch inline (because you're considering running spot lights off the 12v run), you don't need a relay. Had you switched, it, I'd recommend using a relay since you have the potential for high draw. Not sure what gauge wire you used for your run, but based on the wattage of your spot likely being high, I hope you used 14 or better. I'd keep a close eye on the wiring when using the spot and see if the heat gets high. This would be an issue regardless of using a relay or not because of the draw to the 12v socket. Just keep tabs on it and don't leave it on for long periods if it gets real warm.
You included an inline fuse in your scenario, so you should be good. Even had you wired a relay into your circuit, you would still have wanted to use an inline fuse, so you should be good.