The fact your using ROHS solder means this may be harder than normal as lead free needs higher temps,
The main things your going to face are mainly from the layout stage of the pcb, as opposed to the soldering, e.g. how is the board secured, are components mounted over a flex / sag point, could the weight of a component flex the board, etc, as the main issues faced with ceramic caps come from thermal and force cycling,
For thermal cycling, say you had one side with a flooded ground plain, and the other side loosely placed 10 thou traces, the board would change between concave and convex slightly as the ground plane expanded and contracted, so the more towards the center of this interaction, the less stress on your components.
For force cycling, are there any board mount connections that are likely to have a large number of connection cycles, if so is the board supported so it will not flex if someone doesn't insert / remove it dead straight, etc
For component spacing, when parallel, i prefer to leave a 10 thou gap between parts, to leave wiggle room (say 1 cap fails, and its pinned in place by its 2 neighbors, which are pinned in place by there neighbors, etc), when series, I've gotten into the habit of leaving a gap large enough to easily fit a solder iron tip between, it may not be needed for re-flow, but it has been a nice rule of thumb that i have worked off for a while now,
For heating with a toaster oven, you will have hot spots, your massive rows of caps will take time to heat up meaning everything else will cook before they have re-flowed, meaning a longer pre-heating period may be required,
If you want to maintain there original capacity, start by getting the right dielectric, as some are plain horrible, then worry about what might crack them off the board, and then you can worry if your solder fillet was to NASA standards (this sounds like a joke, its not, its a very good guide)
Also just because its bulk power doesn't mean its not KHz / MHz pulses, and frequencies, it ties in with decoupling, but even if your just turning on and off an led, a mechanical switch can easily create a current pulse at a few Mhz, as can a regulator or transistor,