It's fine. I've done it numerous times on SQ and other airlines.

You can buy wine at Duty Free stores at Changi. Prices are not exorbitant but not much of a bargain either. If you know what you're doing, you can find much better prices in the US.

If you short-check your bag at SIN, you will have to clear Customs at SIN and thus need to be mindful of the import limits for Singapore as well as Indonesia. If you check your bag all the way to DPS, you won't need to worry about Singapore Customs and will only need to worry about your back with wine inside being exposed to heat while in transit at SIN. Overnight temps should not be a problem at all, say if you're arriving on SQ 15 and departing on the morning flight to DPS. Panicking types might be more concerned about a long daytime transit. I don't know if bags in transit at SIN are kept in temp-controlled environments, but I assume they're not.

Check the thread on Bali Visa and Customs Expediting services in the Asia forum for information on customs clearance at DPS.