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When I last filled my tank to full, fuel poured out on to the ground under the car. It leaked for a while and then stopped. I have since filled the tank to three-quarters and nothing leaked.

What could be wrong? It is a Honda Accord 1998. Will it be an expensive repair? Is it dangerous just to half fill the tank and ignore it?

Depending on the amount of fuel spilt and the exact circumstances (eg; was it on a level surface, had the auto-stop already triggered a couple of times on the pump etc), it is possible the filler neck was overfilled and the fuel was from an overflow. However, for safety reasons, there is a vapour trap fitted and it is a little unusual for fuel to spill onto the ground.
Therefore, it is perhaps possible that there is nothing wrong, and just that it was overfilled, although it may be that a breather hose or some part of the breather system has come adrift or disconnected or even that the tank sender unit or pickup gasket is leaking.
As to the last part of your question, it really depends on the cause and whether it is a fault or not as to whether it is dangerous. Obviously, a raw fuel leak is potentially dangerous, as is fuel venting to the atmosphere (whether it is full and leaking or not), but if it was just an overflow from being overfilled, and not an actual leak, it may be fine.
I'd suggest having a mechanic look at it, although it may be hard to find the problem unless it is happening at the time, due to fuel evaporating quickly and not being able to see where it is coming from.