Her overall behavior suggests that she loves you.  Her occasional hissing
while you're playing is probably an expression of displeasure over a touch
or a movement.  I wouldn't worry about it.  If she exposes her belly to you
and allows you to rub her, she definitely trusts you and wants you to touch
her.  I think the hissing is all in play.

My kittens are playing with each other at the moment. The female hisses a
lot but carries on with the rough play instantly.
Some cats hardly ever hiss, some do it all the time.
And ... you can hiss back - watch the reaction!

I never ever rub a cat's belly, always the back and the chin.
I've seen people bring their cats into the shelter because they bite,
but I believe it was because they just played the belly game.
I assume only one cat in a hundred really bites, or even less.

Carola

I believe Wayne is spot on about the movements in relation to hissing.
Usually when I pull away from a Love Bite it may startled him and
he'll make a funny hiss or miaow noise.

Cats are very temperamental.  They have different moods and can change
in the blink of an eye.  When they have just come in, and before you
are about to feed them they are probably at their most affectionate "I
wonder why?" :)

As far as our cat goes I try to understand him by trying to think the
same way a cat would think.  There are different moods and modes for
our cat.

3 Affectionate Mode
1 Food Mode
3 Sleep/Cat Nap Mode
1 Resting/Leave-Me-Alone Mode
2 Sphinx Mode
2 Cleaning Mode
1 Predator/Ambush Mode (both the same)
3 Play Mode

3 = okay to stroke. 2 = stroke with caution. 1 = don't stroke.

As far as the bites go, I believe a cat can bite you in one of two
ways. The usual way is a "Love Bite" which is just a sign of
affection. This doesn't really hurt. Then there is the "Real Bite"
that hurts  when he is cross and you need to back off. The Real Bite
has never happened to me, just play bites.

As far as the paws go, there are a few different actions he can do
here as well. There is the Play Paw, this usually occurs just before
feeding and he has his claws withdrawn, and is usually putting his
paws on you or usually the packet of food you have and are about to
feed him.

When our cat has just come in after being out all day, he is usually
miaowing a lot. When you stroke his head using your whole hand he
loves it and usually collapses in a heap on the floor and rolls
around. Your hand usually follows and to stroke him on his belly. He
will then play bite your hand with his teeth and paw at you with his
rear paws.  This doesn't hurt (usually) sometimes you may get a slight
accidental scratch though.

Then besides the play paw you have the attack paw. Sometimes this is
accidental or on purpose. If he is in Predator mode and chasing things
you are moving across the floor or behind curtains, sometime he may
scratch your hand.

The way he bites your hand for fun and also paws at you with his rear
paws, is also one way he can terminates real prey outdoors or pretend
prey that you are moving across the carpet of your living room.

Cats have a way of making you feel guilty even if you have done
nothing wrong. They will give you funny looks.  They also often have a
habit of following you wherever you go (when you don't want them to),
which can sometimes be annoying especially if it's the bathroom and
they just sit there staring at you :)

As far as Affectionate Mode goes, this works both ways. When our cat
comes in and is hungry or even just happy to see you, he will jump up
on my desk and rub his head against mine and make lots of purring
noises.  He responds if I say his name softly/quietly and if I repeat
it every so often.  Also you can try making purring noises back.  Our
cat really likes it though when I use my whole hand to sort of stroke
him in a brushing back/grabbing motion over his whole face eyes and
head.  

Also when he is just about to have a proper sleep/cat nap or is just
resting, he loves it if his head is stroked firmly just from his
forehead right about his eyes to just about where the ears are. He
loves this.  His eyes start to shut as if he has been hypnotised.  

Sometimes you can use two hands one for the head and the other one
down his neck and along his back.  I generally don't do the belly that
much. His head is the main area where he really loves to get stroked
and I usually just the part of the hand from my knuckles to the middle
of my fingers to stroke him.  If you try this when your cat is in an
affectionate mode I believe there is a good chance she will promote
you to her favourite person :)

Our cat can often switch from a playful affectionate mode to an ambush
or predator mode.  Once you can understand almost instantaneously what
mode she is in, it will be rare that you ever get scratched. I've not
been scratched once this year so far. The last time was December last
year and that was my own fault. He got me through the curtains and put
a scratch right across my hand.  He did think it was something to kill
though :)

John