I don't think one or two cm over a year is a problem, but if you're losing it at an increasing rate, that is bad. Keep an eye out for puddles or drips. My TS 1.8 had a leaking radiator because an air conditioning sytem part behind the radiator rubbed a hole in it.
The part I'm referring to is a cylindrical alloy object about half the size of a coke can, located on the outlet hose that comes out of the bottom drivers side corner of the air conditioning condenser/radiator. It usually sits in a bracket a few cm behind the engine radiator down the bottom drivers' side, but mine had slipped out of its tin bracket and proceeded to gradually wear a hole through the radiator fins. I noticed small losses from the reservoir initially and didn't think much of it, until it left a puddle under the car and I managed to locate the source.
Because the astra radiator has plastic end caps, I couldn't get it re-cored, and had to get a new one, which set me back about $400 fitted. I might have been able to get the radiator fixed if I'd noticed it before it made a decent hole. I've zip-tied both ends of the part now so it won't cost me any more unneccesary coin wastage.
Hopefully you don't have this problem - good luck!
If you want me to take a few pictures of it for reference PM me your email or I'll post em up or something for all yo a55es