You can run 255/55VR18, 245/55VR18, or 255/50VR18 tires on your OEM wheels without any safety issues. Be sure the load rating is at least 99 and do not drop down to lower speed ratings like H. Some minor rubbing in hard cornering may be found for the larger of those, but if you are doing that with your family in the car... then you really aren't that concerned about safety are you?
Your safest bet if you don't want to take any risk is to just get the the OEM tire size. The Bridgestone Dueler H/L Alenza is your tire of choice for this size.
CAUTION: I would avoid wheel spacers as they change the way loads are transfered from the wheel to the hub. These OEM wheels are desinged to fit over a feature on the hub so that the wheel bolts are only in tension (no shear loading). If you install spacers, this function is lost and the wheel bolts now have to carry tension AND shear loads. They were not designed for this. This is a REAL safety issue.
To move to wider tires without any clearance issues or spacers you will need to get different rims that have a greater offset from the hub. Just be certain thay engage the shear feature on the hub in the same way the OEM wheels do, or you will be placing the botls in shear again.
Sounds like you know what you are talking about! After much research, I am planning on going with the Michelin Latitude HP 255/55-18 104H, which is what comes standard on the Acura MDX. It has the closest overall diameter of any 255/55-18 tire so it makes it closer to the stock OD. I never go over 75MPH so I didn't think the H mattered and the MDX seems like a very similar SUV.
Go here and plug in your sizes and you'll see that with the OEM 235/55-18, the diameter difference with the 255/55-18 is not recommended. 255/50-18 is a better fit.