If your really concerned get them o2 cleaned and serviced at the same time? Personally I only get regs serviced when they break, so if you can vouch for where your regs have been you could use them. However the o2 service packs are the same price so the cost for o2 clean AND service should be exactly the same as a normal service IMHO well maybe a couple of quid more. The only difference comes in when they use a *more expensive* chemical i.e. biox in the ultrasonic cleaner, o2 lube i.e. christolube and they wear some gloves
. In all honesty you could do it yourself at home there are plenty of manuals about it online. Look up vance harlows book.