I am making a 20x40" coffee table, and will use (for the first time!) table
top fasteners. I have built several other tables, but have run the grain on
the sides vertical rather than horizontal so I could just glue the top on.

1) Since it is winter and the wood is presumably as small as it gets, I want
to just barely put the fasteners ( the offset clip things) in the slots on
the front and back, to allow room for the top to expand. The sides can be
in close, since they will only have to accomodate sideways motion. Right?

2) Does it matter how tight I have have the top to the apron, or do I have
to do it loosely to facilitate movement?

3) Since the wood should grow by 1/4" at most, could I glue the front and
use the fasteners on the sides and back? Or is that pressing my luck?

I wouldn't glue it anywhere. You're asking for trouble if you do. Use
the clips you mentioned or make your own. I usually cut a 3/8" dado on
the inside of the apron 3/8" from the top, forming a groove which runs
all the way around the inside. Then I crosscut several 1" lengths of a
3/4" hardwood board, and I rabbet it to form a 3/8" tongue which mates
with the groove on the inside of the apron. I crosscut the 1" strips,
rather than rip them, so that the grain runs perpendicular to the
tongue, making it stronger. Then, to attach the table top, just set it
on the apron, place a couple of your homemade brackets on each side and
screw them in to the bottom of the tabletop. The depth of your dado
(and depth of the tongue) will depend on what kind of movement you
expect out of the wood. You mentioned that your tabletop should grow
by 1/4" at most, so I'd make them both about 1/4" or 5/16" deep and
screw them into the tabletop about 1/8" away from the apron. If you
think it might grow a little more than that, you can oversize the screw
holes in the brackets so it has room to slide a little if necessary.

Funny story: The first time I ever made a table, I did it in the
summer and gave it to my in-laws for their kitchen. It was fairly
small, maybe 40" wide or something like that, made of maple. Like an
idiot, I fixed it directly to the apron without allowing for movement.
5 months later, on Christmas day, my in-laws had a get-together at
their house. This was in Maine, and that particular Christmas I
remember it was ridiculously cold, 32 below zero when we got up. Well,
my mother-in-law cranked up the wood fire and got the place roasting
inside, so I'm sure the air was a dry as the Sahara. Sometime in the
afternoon, we were all sitting in the living room when all of a sudden
we heard "POW!!!!", like a gunshot from the kitchen. We all ran in to
see what had happened, and it turned out that the tabletop had split
right down the middle! There was probably a 3/8" gap in that sucker.
Took me a couple of weeks to get everything fixed and reattached
correctly.

we heard "POW!!!!", like a gunshot from the kitchen. We all ran in to
see what had happened, and it turned out that the tabletop had split
right down the middle! There was probably a 3/8" gap in that sucker.
Took me a couple of weeks to get everything fixed and reattached
correctly.

Good to know things like that actually happen; it makes the precautions
worthwhile.

1) Since it is winter and the wood is presumably as small as it gets, I
want
to just barely put the fasteners ( the offset clip things) in the slots on
the front and back, to allow room for the top to expand. The sides can be
in close, since they will only have to accomodate sideways motion. Right?

Yes.

2) Does it matter how tight I have have the top to the apron, or do I have
to do it loosely to facilitate movement?

Not really. I torque it just to pull everything flush. I see no benefit to a
death-grip.

3) Since the wood should grow by 1/4" at most, could I glue the front and
use the fasteners on the sides and back? Or is that pressing my luck?

You could, but that would make it considerably more difficult to do a repair
should one be required at a later date. I just don't see the benefit.

-Steve

Good to know things like that actually happen;
.........


Sure, go ahead, revel in his misfortune

;-)