The best time to negotiate is when something is marked down. The mark down price means they want it to move. The price is already on the slippery downward slope and you can nudge it some more. Asking is free and never hurts. Do not insult them with a price, offer a reasonable lower price, you never know it may work. Many years ago I bought a used Ford Taurus with a 4 speed manual (4 cyl not an SHO ). It had been on the lot for a while and not a lot of folks can drive a stick. I did insult them with my first low ball price figuring we might meet in the middle. The sales rep wandered off and came back saying they would accept the offer. I was stunned for a few seconds then happily made the deal. Decent car for the price and time we had it.
I saw a stove at one of the big box stores that had the big center burner my wife wanted. It was white a lot of folks wanted stainless. It was marked down. Retail stores rarely negotiate prices. An assistant manager just happened to walk by and asked if we needed help. I asked if we could get a better price and he gave us another 10% off. Worst he could have said was no, but asking is free. Do not be afraid of making a counter offer.