It definitely helps to figure out what kinds of portion sizes you should be eating. You can roughly judge how much of each kind of thing you're eating at each meal.
I was pretty surprised when I found out that I'd been eating much more protein than I realized at each meal.
There are different ideal food portion sizes depending on what kind of diet you're trying to follow, of course. The Atkins diet, for example, advocates having much more protein than carbohydrates, whereas most of the time people are told to have more carbohydrates than protein.
And in general it's good to know how many calories are in each size portion of food, just so that you can keep an eye on how much you're eating.
Counting calories can get tedious after a while, but if you can roughly judge each meal at a glance, you're more likely to keep it up. If you add that to judging how much of each different kind of food you should be eating, you'll end up with a decent diet.