Eurocontrol guidlines are to switch 'On' your Mode S Transponder during taxying so that Airports with ASMGS can receive the enhanced info and provide better montoring on the ground, however leave your TCAS
off until lining up (i.e. use XPNDR mode or equivalent).
The operation of many TCAS systems in a small area (like a busy airport) can cause saturation of the RF frequencies in use and require the TCAS unts to work a lot harder in decoding and responding to all the replies about. The use of such kit on the ground, where it is not
really needed, has the potential to degrade the operation of TCAS onboard airborne aircraft.
So whilst it is nice to see traffic flying about as you taxy/cross runways etc., it is a bit anti-social. Leave it until line-up to have a quick look down the approach path, and even then remember that although it is unlikely to show traffic where there is none, it may not always show traffic when there is!
Lots more information at:
http://www.eurocontrol.int/airports/...ts_asmgcs.html
http://www.eurocontrol.int/airports/...leaflet_v2.pdf