Should I have my gearbox serviced?

[triumph2.5pi]

Hi all
I have a 2000 2.8 Sport 2WD, 102k miles with a full service history, although there is nothing to specifically say the gearbox itself has been serviced. I read somewhere that if a gearbox has done over 100k and not been serviced it's better to just leave it as it is? Is that really the case? Or should I have it serviced? If so, am I right in thinking it should be serviced every 20k miles? Thanks for any advice :-)