Your comment reads as if you equate 'BGA' with 'SMD', a BGA is a ball grid array with up to 1,000 tiny pads that have been pre-dipped in solder. Your toaster isn't going to work.
If you screw up the solder job, you can use a hot air gun to remove the BGA part, reball it (using a stencil, fairly cheap if its a common package), then try reflowing again.
If you practice for a few hours on scrap electronics, you can get good enough at it.
It is a bit rude to assume I don't know what a BGA part is.
Anyway, if you do this stuff often enough then I see no reason why you wouldn't get the proper tools, a rework station and an actual reflow oven or something with a PID controlled heating element would make your life so much easier. Working with bad tools would drive me nuts.
The reason the larger BGA center themselves is as soon as the solder goes fluid there is a lot of accumulated surface tension trying to reduce the size of the bridge and that will center the part all by itself. For that to work properly though everything has to become fluid more or less at once and stay fluid until the part has shifted to the right position.
As for the reflow you can actually get more consistent results with the toaster oven - It just won't be able to handle the volume of actual production. Whatever you do just don't try going "semi-pro" and getting one of those IR ovens from China. Stick with the $40 walmart special. The toaster oven when heated slowly is much less likely to have hot and cold spots. Stenciling and placing parts take up a lot more time and are much more error prone.