Well technically you should intercept the LLZ before commencing descent on the G/S, and if the vectoring by ATC is good you will always achieve this. If flying a full procedure (i.e. an arc) the lead bearings/radials will have been designed for you to satisfy this requirement.
As for when to descend, well ideally you should intercept the G/S from below, but that doesn't have to mean you are level before intercepting. For noise abatement and fuel savings, its preferable (IMHO) to reduce your RoD and allow the G/S to drift down before intercepting.
For example, let's say you're in an aircraft that is grounding 140kts. To achieve a 3 degree G/S you'll need a RoD of roughly 700fpm (5 x 140). Let's say ATC clear you from 5000ft to 3000ft and give you a radar heading to intercept the LLZ and clear you for an approach. If you are below the G/S at this point, and you descend at greater than 700fpm, you'll never catch the G/S. Anything less than 700fpm and you will catch it at some point. So really it's up to you to ensure that you reduce your RoD sufficiently to ensure that you intercept the LLZ before the G/S. If you can achieve all of that without levelling off and having to open the taps, then it's a bonus!
Hope my drivel makes sense, i'm off to the pub for beers and 2up!
TL