I'd agree - so long as you are consistent and match the temperature of the new water with the old, the bigger the changes the better.
Another 'cheat' to get a syphon going is to put your finger over the 'out' end, invert the rigid plastic part that goes into the tank (so the opening is facing upwards, and fill it with water. Lift it up to eye level, and take your finger off the other end so that water flows into the hose (watch where you might squirt it). Once the plastic part is nearly empty, block the hose with your finger again to stop it, and put the plastic part back in the tank and let it refill with water (make sure there are no big bubbles trapped inside). Keep it underwater, point it back down at the gravel, take your finger off the hose, and hey presto!