Is this an upright or a cocktail?
Normally I'm one who advocates keeping an isolation transformer in place even if the monitor you put in doesn't need one. It's 'original', it's extra protection in case you do something dumb later between the power input and the monitor, and great if you decide to drop back in a iso-requiring monitor later and forget that you don't have the iso. Yes, it costs a tiny amount in additional power usage and generates a small amount of heat.
But in the case of a cocktail -- I would actually remove it because it will help with preventing monitor distortions due to the proximity of the transformer to the neck/yoke.
A lot of people blame the card edge connectors or fuse blocks in Midway cocktails in creating hum bars on the monitor, but I've found that the iso or the step-down transformers that are inches away from the tube can also affect it.
Also for those doing this in a cocktail -- remember each bulb in the game uses about 1/4 amp of power, so you will have 1.5 amp draw if you have all 6 connected (2 per player, 2 per coin door if Ms.Pac style dual door, or 1 per coin door if a single entry original Pac door droping it to 1.25 amp) Make sure you have sufficient amp service on your switcher for either the 5 or 12 volt line for both the gameboard AND the lamps together, depending on the voltage of those lamps/where you plan on connecting. Put a multi-meter in place to measure the draw of either part to be sure you don't overload the switcher.