Use the gas assist all the time, with the chimney. I brush ash off of it, when I have the charcoal grate removed for quick sweeping out. A vacuum's hose placed over it does a good job of sucking out ash from the tube, if needed (usually not needed).
My advice is to use it to get started, but run it no more than a few minutes. Let the chimney take over from there, unless you like running to the store for propane. Plan your cook 10 minutes early and you won't miss the extra time needed to get coals totally lit.
The other reasons I don't run the gas very long is because it WILL slowly ruin the charcoal grate (no matter what ... ) and because if you leave the gas on, you'll get big flames out the top of the chimney without them actually ashing over faster. It's like they're getting so much air flow, they go to flame, when what you want is ashed over. Does that make sense?