Second stages normally need service as in most designs the second stage seat is pressed against the second stage orifice constantly - even when the reg is not being used.
Some second stages use a poppet design that relieves this pressure against the seat - but they are also a lot more complex and benfit from an annual service anyway. Plus the system is more or less open which allows creepy crawly things to find their way into places where you do not want them. Some other designs use a key or pin to keep the purge slightly depressed to accomplish the same task of keeping the seat off the orifice in a lower tech way. But these do again leave the system open which can cause dust and insect issues.
First stages do not have shelf life issues, but are more prone to corrosion problems in inadequately rinsed, so skipping a year may not be cost effective in the end.
$85 for labor on a reg set is a little on the high side, but not really outrageous. I am not sure what you pay to go on one or 2 tropical trips per years, but we average $1500-$2500 per 7 day trip from where I live to anywhere nice. So unless you are getting really good deals from your travel agent, $85.00 is a drop in the bucket campared to the prioce of the trip and more importantly is extremely cheap insurance to ensure the reg will work on the trip.
I do not recommend a service right before a trip as if an adjustment issue or other problem occurs, it will be on the first dive or two after the annual service. And issues do develop from time to time even with the best techs and the most careful service. Most are a 2 minute fix, but it's hard to do when the customer is 200 miles away.
One option for the infrequent diver who does not want to shell out for a full service is to get a flow check. This will involve an inspection and flow test on the bench to ensure everything is operating properly. Cost is around $10-$20. It offers cheap insurance without having to worry about slight freeflows, etc caused by a new seat breaking in on the trip, or a variety of other things and is a better option for the last minute traveler