We are in the same situation as you, OP. DW is still working but won't be in a couple more months. But we've been living on our ER budget for 2 years now. Year 1 we slightly overspent the budget to tackle a planned major house renovation (exterior siding, new windows, major roof repair).
Year 2 we are running about 25% under budget (in spite of 8 weeks of international travel - about all we can squeeze in with 3 kids, 2 of which are in school). I'm mostly doing your option (a) - noting it and moving on.
But for 2016 I'm increasing my target budget by about 25% to hopefully get us to live a little larger and spend more. The new and improved larger budget still represents 3% of our actual portfolio (after carving off set asides for kids' college and some other fun stuff).
I'm not worrying about carrying forward our 2015 cost under runs, but I'll certainly feel better about busting the budget in the future if a major unexpected expense pops up or we tackle a major replacement item (HVAC, roof, new(er) car, etc) in the next few years.
I certainly don't see a need to spend the money frivolously, but instead deploy it in your ordinary budget throughout the year. For us, that's meant trying some pure luxury purchases at the grocery store (fancy cheeses, beers, wines, champagnes, caviar, etc) and booking another cruise and searching for a 3rd winter cruise for just me and DW. It also means we can replace our (rarely used) aging car(s) in 2016 and not worry about the spike in expenses for 1 year.
Retired in 2013 at age 33. Keeping busy reading, blogging, relaxing, gaming, and enjoying the outdoors with my wife and 3 kids (4, 10, and 11).