MacBook Pro 2018 & LG UltraFine 5K Display Power Comsumption
For MacBooks (including Pro), if the requested voltage based on current system load is higher than the power available, it will try to make up for the difference by drawing from the battery; if the requested voltage is lower than the available power, it will use the extra power to recharge the battery until it is full. 

For using the 85W charger on an 87W MacBook Pro, the difference is so small that it is virtually unnoticeable. In rare cases, your battery might be losing a small amount of power slowly when the machine is under max load. In most case, it will be just fine.

Note: The LG UltraFine 5K is made for MacBook Pro 15" and MacBook Pro 13", and you can see that MacBook Pro 15" has the best feature support. So it is safe to assume that everything, including the power delivery over USB-C, is highly optimised.