I think we need to define what sandbagging actually means.
2 definitions:
Defn1: loading the car up with a full tank of fuel and less than optimal setup, while having the driver drive as quickly as he can, is classified as 'sandbagging'. Lets assume that the team only run the car with near full fuel loads. This would be very very dangerous. The team will end up setting up and developing the car which performs great on heavier fuel loads (remember, all their data is based on full fuel loads, as they have not tested the car with low fuel loads). The idea must be to produce a car which performs well at all fuel loads and not just a heavy or light fuel load. Hence, assuming that running the car with heavy fuel loads is defined as sandbagging - would be a very dangerous practise and would only be practised if the team believe their car is miles ahead of the competition, which in turn means they don't ever have to bother testing the car with light fuel loads.
Defn2: the team set the car up to perform fast (low fuel loads, fast set up) and slow (high fuel loads and inappropriate setup). However, now the driver is instructed to go slow on purpose (ie. hide the true performance of the car). This practise would be pointless and risky, as the team will almost never get a true reflection of just how fast their car can go. At some point, the team will need to estimate the true capabilities of their car, which in itself is risky. Any improvements/atlerations to the car would be based on assumptions and estimates. Developing the car in this way could be disastrous, as the first time the driver drives the car on the limit (at the first race weekend), the car may not perform as predicted, when at its limit. Hence, asking the driver to "hide" the true performance of the car or "sandbag", is very very dangerous.
Whichever definition of sandbagging you use, sandbagging, gives you virtually no advantage and can in fact leave a team with no idea of just how fast their actually is and how it performs when at the limit. Sandbagging (using both the above definitions), may also have the effect of reducing morale, within the team as they see their cars always slower than the opposition.
In all the years I've watched F1, I have never seen a team "sandbag" and in general, if a car was fastest during most of pre-season testing, then it was also fastest in the first few races of the season. The perfect example was BrawnGP in 2009 - fastest in pre-season, fastest in the first few races of the season.