According to Richard Turner's "A Directory of Scottish Provincial Silversmiths & Their Marks" the scarcity is rated below. However I don't know how definitive this list is. What I do know is, that you will certainly find a lot more silver items with a Glasgow assay mark than you would with a Perth mark for example.
Scarce: Aberdeen, Dumfries, Dundee, Glasgow, Inverness, Perth
Rare: Arbroath, Ballater, Banff, Canongate, Elgin, Greenock, Iona, Montrose, Paisley
Very Rare: Cupar, Ellon, Fochabers, Forres, Keith, Nairn, Peterhead, St. Andrews, Stonehaven, Tain, Wick