On a high-pressure boiler ...

On a high-pressure boiler you will likely need fully deionized water as feed.  Depending on the daily volume of makeup water, an RO in front of the DI tanks may be justified.

For a low-pressure boiler this decision is typically justified by weighing the capital expense and maintenance of the RO (with necessary pretreatment) against the recurring costs for DI tank exchanges.

In regard to a previous comment, I'm not sure what would drive one to place an RO system after an ion exchange system, unless the ion exchange system in question is simply a sodium cycle softener.