Depending on the livewell pump you currently have, it might be inadequate for washdown duty. Instead, you might want to split the water line after the seacock and install a dedicated washdown pump. Check the specs on your livewell pump - a washdown is usually in the neighborhood of 4 to 5 gpm.
But, to answer your question, tap into the line where ever it's the most convenient for you. As long as it's after the seacock and before the actual livewell, it doesn't matter.