Heat can damage the varnish finish and cause serious cracks in the wood. Hot temperatures cause the wood to expand, dry out, and crack. Very hot temperatures will cause the glue which holds the instrument together to fail. Cold temperatures cause both the wood and varnish finish to contract and crack. Wooden instruments will dry out if the air lacks humidity.
Instruments do best when the humidity in the air remains stable.
Use a dampit in the instrument's case or run a humidifier in the room where the instrument is kept to prevent the instrument from drying out and cracking. For polishing the wood surfaces of your instrument, do not use household furniture polish. These polishes contain wax, which is not good for the varnish. Here is a polish recommended by a violin maker. For best results, use a varnish cleaner or soft grit rubbing compound polish like that used for waxing cars.