All aircraft I've been on - several different manufacturers - have advised to release the parking brake. Airbus said that it was advisable to release the parking brake as the heat buildup in the brakes caused the hydraulic fluid to break down and become acidic. This contributed to corrosion in the calipers/pistons.
However, at Heathrow some gates have a slope away from the building - presumably for water drainage - and we are required to keep the parking brake set until unloading is complete. Maybe this is a BA procedure for this airport only?