Some of the information you've gotten is bad. This comes from the
plumbing newsgroup. First of all, don't put a washer between the head of
the bolt and the rubber washer. The brass bolt head and brass washer isn't,
of course, a watertight seal.
Secondly, some toilets come with a second rubber washer for under the
tank, but most don't. It sounds like you installed it exactly right; leaks
just sometimes happen. First of all, does it leak constantly, or only when
you flush it? If it leakes constantly, and the water's coming from
somewhere between the tank and bowl, then it's either the bolts (and rubber
washers) or the flush valve (the big hole in the middle) or the tank might
have a hairline crack. Test the big nut that holds the flush valve to the
tank to make sure it's snug. The smartest thing to do is to just reassemble
it again. Maybe the second time's the charm. If the tank is cracked, you
might not be able to see it. One trick would be to color the water with
cleaner or food coloring to detect the location.
If it only leaks when you flush it, then it's the connection between the
spud gasket (that foam rubber thingy) and the bowl. Again, just reassemble
it. The second time's often the charm.
And lastly, if it's a plumbing question, just ask the plumbers instead of
the DIYers. It'll save a lot of time and protect you from misinformation.