If you're pushing back you can't really justify starting #1 or #2 first based on which one is farther from the jetway, but here's an important thing to think about...
Boeing used to power A hydraulics with EDPs (Engine Driven Pumps) and B hyrdaulics with EMDPs (Electric Motor Drive Pumps, also, I realize some aircraft have more systems in general and there are also AUX and STBY systems on others, but let's stick to the basic 2 system layout). This was fine until they came up with the crazy idea of only slinging 2 engines under a plane. The first 737s (100 and 200) did have this same layout of pumps, but starting with the 300 Boeing began putting one of each on each system, for greater redundency.
What does this mean to you?
Well if we look in our Boeing books you'll notice that A hydraulics power nosewheel steering and B powers brakes (among other things). Being under tow/push you have no use or A system (in fact it can be quite dangerous given the circumstances on the ground to pressurize the A system up during push). You also have a lot of use for B system, namely, the Brakes. This is part of the reason starting with #2 is a good idea. But, there is one more, turning the A pumps off during push is part of many checklists, but having them selected off and actually being off are two different things, you see. EDPs on many Boeing are "powered off", which means if that if you were to lose power during pushback with an engine which powers an EDP running (#1 on 733 and later) you would pressurize that system, with potentially dangerous/deadly results. Losing power with #2 running wouldn't do a thing other than keep B system pressurized when the EMDP goes offline due to the same power (AC) loss.