It doesn't hurt your profits at all.
.... until the other guy declares war and you lose all your trade routes at once.
Personally, I recommend spreading trade routes out amoung all the other races. It gives bonuses to your relations that can come in handy when trying to avoid wars and also means that any one race declairing war on you means a much smaller hit to your trade revenue. It does depend on the exact situation in the game, however.
And one other thing I've started doing is buying the AI's freighters. They're a lot cheaper than most of their ships, and many times the money they're willing to give for my tech is capped, but tech is worth more than they're offering. I'll ask them to throw in a freighter and they'll do it essentially for free. Half the time, the freighter is already near their homeworld, so I start the trade route that turn. You don't get to chose which of your worlds is the terminus, but its a quick way to get trade routes started in the early game, and if you want to send them to a different world later, you can always make new freighters there, cancel the route, and make the new one.