if they are not being rendered on top of the enemy as a seperate sprite, then yes, I believe slightly more centered would be good. Slight variation is good, just keep in the mind the more unique the explosion the quicker it will become noticably repetitive.
here are the options, in order of preference (mine)
1. Seperate explosions, rendered in a random fashion at run time, allowing variable / random size, speed and position
2. Several pre-drawn explosions, randomly selected at render-time to add a bit of variation
3. Fairly centered/symmetrical/logical (eg. explosion starts on engines, then cabin explodes, then chassis)