Right now I hate Waves, since I'm typing half of this behind their advert.
Since programming DSP is something I do for a living, I think I'm reasoably qualified to answer this question.
There are two reasons why a digital summer might saturate, the first is a hardware limitation in the numerical format, the second intentioal.
Let's deal with the first one, the numerical format. There are three main ways of storing numbers in digital form, integer, fixed point, and floating point. Now for the purposes of this discussion we can lump integer and fixed point together (in fact integer is a special case of fixed point really).
In the case of a fixed point addition, the range of numbers that can be stored in n bits is +/- 2^(n-1) (well the positive range is one less than that if anyone wants to be pedantic).
If you exceed that range then depending on the way the adder is implemented it will either saturate, or wrap (really nasty). Avoiding this is conceptually not that different from analogude design, you can reduce signal levels at certain points to prevent saturation, but the more you do that the less signal to noise you will get, so it's a tradeoff, or you can use double words which give you masses more headroom with no quality loss, but that uses much more processing power (think of that as a bit like an analogue designer having the option to use super-duper-mega-high quality components but them being very expensive).
But actually there aren't many systems which use fixed point processing that I know of. The only ones that spring to mind are Pro-Tools, which these days uses 48 bit words in the mix buss (that's what they introduced when they started calling it Pro Tools-HD) and SAW on the PC, which I think uses 64 bits in interim calculations and 32 bits for storage and busses (note that 32 bits lets you add 256 24 bit signals before you could possibly reach clipping in the worst case).
Most systems on computers use floating point, and in the case of floating point the numbers that can be stored are huge, so you're not going to get accidental saturation.
So that leaves us with the intentional saturation. The thing is that your final output format, be it a DAC or a CD/DVD, is going to have a limited range, so the numbers that are fed to that are going to have to be kept within that range, but avoiding that is just a case of setting your output level correctly.
All of the above basically adds up to the following...
in modern DAWs, so long as you don't see any clipping lights, you shouldn't have anything to worry about.