Lateral trim is indeed important. Although there are no "limits" prescribed for most light GA aircraft you can get a feel for how an imbalance can affect flight in some of them.
This is actually a fun exercise in a Cherokee:
- Fill one tank to the tab (~16 Gallons, 96 pounds of fuel)
If you're solo this should be the right tank.
If you take an instructor the tank on the side of the lighter person.
- Fill the other tank completely (~25 gallons, 150 pounds of fuel).
If you're solo fill the left tank.
If you take an instructor fill the tank on the side of the heavier person.
- Start, run-up, take off, and climb out on the tank you filled to the tabs.
- Fly around for 30-45 minutes to burn off some fuel.
With typical fuel burns you should wind up with 6-7 gallons, or around 40 pounds of fuel left in the low tank. That's roughly a 100lb imbalance 3 feet out on the wing.
- Turn into any wind, slow to around 80 knots, bring the ailerons to neutral, and wait a few seconds.
In most cases if the aircraft is rigged properly there will be a slight roll toward the full tank (heavy side). You will find you need to maintain some light aileron pressure in order to keep the wings level.
Remember to switch to the full tank after completing this little demonstration.
You don't want the engine to quit on your way home - that would be embarrassing!
A similar demonstration can be arranged with a Cessna 172, or really any aircraft that has a fuel selector which allows you to burn fuel from one wing exclusively on demand, but you should consult the POH and/or a qualified flight instructor about the particulars of the aircraft's fuel system before you go and try this on your own. I happen to know it's relatively safe in a PA-28, but I can't speak for the fuel systems on other aircraft.
The net effect of poor lateral trim is that the pilot (or autopilot) needs to maintain some control input to counteract the aircraft's tendency to roll toward the "heavy" wing.
Large aircraft (and "fancy" light aircraft like the Beechcraft Bonanza) have aileron trim to help out with this, but for aircraft without that particular luxury the pilot needs to hold the yoke/stick to maintain the desired control input.
As others have mentioned, larger aircraft often have valves and pumps to transfer fuel between wing and fuselage tanks to help achieve proper lateral trim.
The best most General Aviation aircraft can usually offer in this regard is a fuel selector that lets you burn from the left or right tanks: You burn fuel from the heavy wing until it's not heavy anymore.
Non-Fuel loads can be managed in a similar way: If they're liquid provide several storage tanks and pump them around as needed (there are gliders that do this with water ballast, though I believe that's primarily for pitch trim). If they're solid (or a liquid load in fixed containers), load them so they're well balanced, and secure them so they don't shift in flight.