If they are peat based then the reason is that once dry, peat is extremely difficult to re-moisten.

And, just to add to Owdboggy's explanation, your tomatoes will just *love* a pint of water, or two. They enjoy being moist and once they flower then loads of food as well.

A pint of water isn't an awful lot for a 1 foot tomato plant in a grow-bag as the water will not all be sitting by the plants roots but will spread out into the surrounding compost.

No it wasn't! Some people make an extra hole in the bag or possibly one at each end, insert a plant pot of cut off bottle, and fill that with water, which then trickles through into the bag. I just tried it and it works.

Makes sense Steragram

I have been hesitant about watering tomato plants much because I thought that you should wait between waterings for the soil/compost to get almost dry (unless the weather is hot). Am I muddling this up with peppers? I also thought that they should not be wet all the time, because this makes the roots rot. I expect that this depends on whether or not they are growing in pots though.

The following applies to my tumbler type, which I am growing in pots. They are about one and a half feet high, and do not have any trusses yet, because I left it a bit late to plant them.
So far this year, I am sticking a long piece of thin wood into the soil to see if it is damp, and I am not watering until there just a trace of moisture on it, even when it goes four to five inches deep. This is working out at about one and a half pints of water a week for each plant. Does this sound too retentive?

I expect this sounds very detailed. The hardest thing I find about growing veg is the watering; it is a double-edged sword in that you can water too much or too little. I have looked for advice elsewhere on the internet, but I never seem to get a satisfactory, conclusive answer. I know that it depends on how easily the soil drains, and the weather, and I know that the leaves wilt when they are too dry - I would think that ideally they should not get to this stage though.
Any more advice is welcome.

I have just seen your comment Steragram. It sounds really good.

You have to watch it though as if you haven't sunk the pot in deep enough which isn't very easy to do after the tomatoes are planted, you might have to hold up the edges of the bag round the pot a bit until the water runs through or it tends to seep up and run out.
Don't stress too much about watering. Better too little than too much - you can see the plant flagging a bit if its too dry - although this doesn't do any lasting harm it can check them a bit but not as seriously as overwatering can. If you pay a lot of attention to how your plants look you will probably be able to tell by looking at them if they are feeling too thirsty. Plants are pretty resilient on the whole.

I grow all my tomatoes using grow pots...those fancy green things that you insert into your compost bags , plant the tomato in the middle ring, then water the outer ring.
When enough water is in the compost, water remains in the outer ring and seeps through as the compost dries out.
Only difference is, I don't use them in grow bags, I use them in bottomless pots that exactly fit the diameter of the grow pots, so you end up with a sealed container that prevents water evaporating, yet allows you to keep it uniformly moist. Same principle as in grow bags, but I prefer using pots as they grow better, in my opinion.

http://i938.photobucket.com/albums/ad225/oldshobdonion/tomatoes27thMay.jpg

That picture was taken on the 27th may, from seeds sown 10th of March. Today, they are nearly twice the size and on their 5th truss. I top up with a little water every two or three days. Haven't started feeding yet.

The watering device that you have sounds very good Scrumpygoat. I have heard of them, and I should use them in future.
Your idea sounds very good too Steragram. It occurred to me that you could punch a couple of very small holes in the bottom of a bottle/container, so that the water trickles out very slowly, drip by drip.

Yes - the only reason I didn't do this was because we don't normally buy stuff in those plastic bottles and we didn't have any. You would have to support them somehow though. Actually you could cut the bottom off and use it upside down instead. They would be more stable this way.