The 'standard' is to put the load on the back but it's not necessarily the best. A rear wheel is inherently weaker than a front wheel because of the dish. Also, a large heavy load over the back wheel tends to make the bike more difficult to handle because the load is above the center of gravity of the bike and cantilevered over that weaker wheel. I find a heavy rear load tends to make the bike way to light in the front end and makes the steering feel 'off', like the front wheel isn't touching the ground. This feeling of a vague front end happens to me when I use a trailer also.

Since you sit higher than the center of gravity of the bike and are loaded towards the back of the bike also, putting up to 60% of a 50 lb touring load on the front wheel makes the bike feel more balanced in its handling. The steering feels more connected to the ground than when you have just a rear load.

That said, you have to balance the load from side to side also. I would never try to use just a single pannier on a front lowrider (sorry Lala ), the handling would just be too goofy. I've lost one of the bags up front on a rough road and it was all I could do to keep the bike from crashing.

But...having a rear rack is nice because you have a deck for carrying large, light bulky stuff like tents, sleeping bags, coats, etc.