IMO it isn't worth the hassle to carry around a bunch of spare parts all the time, especially if your trailer is in good shape and properly maintained. If the bearings or hubs are marginal and you're going to buy new parts anyway, you might as well put the new parts on the trailer so you don't have to worry about it any more.
Another thing I've discovered after about 20 years of saving old parts "as a spare, just in case". If a part is bad enough to pull off and replace, then it isn't good enough to put back on. Throw it away. By the time the new part wears out, your used 'spare' will be rusted, corroded or lost and you will end up buying another new part instead of using the POS you've been carrying around for 5 years.
I carry a spare tire (sometimes) and whenever I buy tail light bulbs they come in packages of two so I throw the second one in the glovebox 'as a spare', but that's about it. When I take the boat out I am not worried about the reliability of my trailer because I already handled that. Preventative maintenance goes a long way.