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I think that the only concern is that if your charging system malfunctions and over charges them, they could conceivably explode. However, even the AGM and maintenance-free batteries have vents.
The caps can be removed from maintenance-free batteries, and water added. They, the battery manufacturers, just tell the consumer not to do it because it goes against the image of being "maintenance-free". I have done it, and have seen it done many times with no ill effects for the battery. They are sealed better than conventional batteries and condense most of the water vapor back into water, and thus retains the proper fluid level in the cells, but eventually, water does need to be added to them or else they will die just like a regular battery that has been improperly maintained.
The AGM batteries have an over pressure release valve that will vent if the pressure builds up enough, and if you run them in a non traditional configuration (such as upside down or sideways), it is imperative that the charging system is operating correctly, or spillage can occur through the vent in an over charging event. It just doesn't happen very often.
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