Should I hold onto my items or use them ASAP?
In a nutshell, you want to try to get the ideal items on your ideal champions. If it’s the early to mid game and you still have time to find these, it may be better to hold on and wait instead of combining for weaker items.
If it’s the late game, however, and you’re close to dying but still have basic items sitting on your bench, it may be your best bet to fight for survival using the combined items they can create.
Remember that item combinations are permanent and there’s no way to get an item back without selling the unit holding it. Another good tip is to use a 1 cost unit to create a combined item on and then sell it, that way you don’t have to worry about accidentally combining with a basic item that your carry was holding onto.