I don't think it will have much effect on how the water flows from the pipes, but I've only had one cup of coffee so far today.
Picture this: What if you installed your bog pipes completely vertical. Slits from top to bottom. As you pump water into that pipe, what happens? It will depend on the size of the pipe and the size of the slits, I guess. If you have a small pipe and small slits, you will have high pressure. Water will come out of the bottom first, but as the pipe fills up due to back pressure, it will come out all the slits. If you have enough pressure and small enough slits, flow out of the slits will be pretty even across all of them. If you have a big pipe with big slits and low flow, all the water will come out of the bottom slits and the top slits will always be dry.
Does any of that even matter? I guess that's what you initially asked. Probably not much. I guess if you're not building any way to backflush your bog you may end up clogging the lower portion of your bog before the rest. But, again, depends on... oh gee why am I still typing.