Since I've had some lords join me and was unable to give them any fiefs, over time my relation with them dropped through the floor. Khavel was the worst, I couldn't get him a fief til he was at -100 with me. Giving him Curaw and one of its village got me up to around -85, but it's a deep hole and since I didn't have 90,000 denars to give him in gifts, it's stuck there.
We're at peace with everyone, but yesterday he rebelled and joined the Sarranids, taking Curaw and two villages with him without any bloodshed. There's now a yellow city in the middle of my kingdom. Very bad. So I would definitely pay attention to your relationships.
I've also had the typical problem of giving away fiefs for strategic reasons (keeping lords' lands close together, so they don't run all over the place looking after them, for example), but that's sunk my relationships with a ton of my lords. A lot of them are negative.
However! The other day I was riding around doing some trading and got one of those messages saying that one of my villages has petitioned me for protection from Harlaus, "making it easier to go to war with the Kingdom of Swadia if I choose to."
I went back to my court, declared war on Harlaus, and pow: my relationship with every single lord in the kingdom jumped between 5 and 10 points. It's a giant help, I'm now back in positive relationships with almost all of them.
So I'd recommend minding your business, do some trading, fight some bandits, hold some feasts - but don't start a war until someone attacks you or you get a message like that. If you're patient, the benefits can be huge.