If you're accurate enough not to pee on the counter then the only thing holding back someone so inclined is the social taboo. It's not like the kitchen sink where stuff regularly goes in there. Does pee harm pipes? Will it make the sink bowl radiate a permanent smell? I just hate wasting water by flushing only pee but also don't like letting pee sit for an excessively long time.
I already do that but I can't always line up my shower schedule with my piss schedule. Sounds like I need to start pissing in my sink one month before my lease is up.