When I had a gas grill with CI grates - I would heat the grill on high for about 10 minutes and then scrape with a wire brush - then reduce to the temp I needed for cooking - for about another 5 minutes.
After grilling - I would crank the heat back up to high for about 10 minutes and scrub with the wire brush (while the meat rested).
One thing to think about with CI grates - they are like a CI skillet ... they build up "seasoning" from being used. And, once the grill is heated up and the grates are up to about 160ºF you've killed any pathogenic bugs you would need to worry about.
There is nothing wrong with your grill/grates showing signs of having been used.
"It ain't what you don't know that gets you in trouble. It's what you know for sure that just ain't so." - Mark Twain
I treat my gas grill as if it was a self-cleaning oven.
Turn it on to max heat with the lid closed and go have a drink. In 10-15 minutes, it's fully warmed up and all the crud from the last meal is turned to ash. Brush it off with a wire grill brush and cook the next meal.