The only time that's likely to be an issue is if you have ASK or FILLIN fields, which will prompt for inputs or, perhaps, DATABASE, INCLUDEPICTURE, INCLUDETEXT, RD & LINK fields, as those reference external files - and updating them takes longer. But not updating them also risks using outdated data... If you're wanting REF fields to update, you'd almost certainly want PAGEREF, NOTEREF & SEQ fields to update, too.
In general, you're better off not using that option with REF fields, especially, as REF fields using them tend to get the formatting messed up if you increase the content of whatever they refer to.

You can use Find/Replace to clear the "Preserve Formatting" attribute from existing fields. To do that:
• press Alt-F9 to expose the document's field codes
• do a Find/Replace where:
Find = ^w\* MERGEFORMAT^w
Replace = nothing
• press Alt-F9 to hide the document's field codes
• press F9 to update the field display.

If you're wanting to force a field to display in a particular way (e.g. Bold, Italics, 18pt, Arial), you can use a \* CHARFORMAT switch. To do that:
• Select the field of interest and press Shit-F9 to expose it's field code
• add a \* CHARFORMAT switch to the end of the field code
• format at least the first character of the field code with the font attributes that you want the field to use
• press F9 to update the field display.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]