more water will evaporate through the tubing when it gets warmer, but in minute amounts. It's not like you're going to have to top up every week
Is 38 degrees hot for water? That depends on how you use the computer. I turned off all fans and let the water temp rise to 42 degrees the other day. Very warm. I wanted to see how far the water would be cooled passively. Now this was while using the pc to surf the net and nothing else. So the component temps weren't that bad, cpu cores in the low 60's, graphics card in the 50's. Computer dead silent. Of course were I to start up furmark and prime95 now, the computer would probably crash quite quickly as the 45 degree water can't cool the components very well.
I prefer to keep the water temp in the low 30's under load, so that the components stay cool. Higher water temps won't necessarily damage anything, but make your system less reliable (crashes), especially if your graphics card or cpu are overclocked.
Also you'll use a tiny bit more electricity if components are warmer. Even though that amount is very very small.