For years, all Shimano rear derailers would work with all Shimano shifters, and all front derailers would work with shifters of the same type (road or mountain). So it was possible to upgrade piecemeal. I've heard that the newer 11-speed has changed the pull ratio, so that will no longer be the case.

The other thing you'll have to watch out for is the number of speeds. Shifters have to match cassette; so if you bought a 9-speed bike, and Shimano then came out with 10 speed, the top of the line was only available in 10 speed. If you're careful about upgrading before the 12 and 13 speeds come out, you can probably get away with it; but as noted above, it'd probably be more cost-efficient to buy what you want now.