If you know about the term gear inches, that’s the measure of gearing most use. It takes into account tire size and your gears and gives you the equivalent size the outside of your tire would be if your gearing was 1 to 1. Or if you were riding an old time bike with the big wheel in the front with no chain the size of that wheel. My touring bike has a range between 17 gear inch and 101 gear inch and most will tell you to stay at least at 20 GI.
There is a nice calculator you can put your information in and get all these numbers back.
http://home.earthlink.net/~mike.sherman/shift.html

Another good read.
http://www.sheldonbrown.com/gear-theory.html

I experimented a good bit with my gears before I found what I wanted and for me I found around 15 or 16 GI was the max lowness I could even ride. My legs were going so fast just to stay upright I would spin myself out. I could climb anything but you are moving about the same speed you can get off and push and at that point pushing felt like less work. So I took things as low as I felt I would ever ride and did it with a 24t and a 12-36 cassette. My crank is a road triple and the lowest you can put on one of them is 24T. as to shifting up and down my rings are 45, 42, 24 and I have no trouble going from 24 to 42 I did add a thing called a chain catcher, there are a bunch of different kinds but what they do is when I go from 42 down to the 24 if the chain overshoots it’s a plastic finger thing that catches the chain and flips it back on the granny. I would recommend one for anyone, I didn’t know if it was working or not but after a few months I could see some signs of wear on it so it had done something. My shift between the 42 and the 45 is super smooth and I will even shift that barely letting off the crank. When you do the granny shift both ways do it as a soft shift where you turn the crank slower than what the bike is moving so there is no tension on the chain and you will be fine. Because the shift is a bit harder to the granny I tried to find a cassette that let me have a good range of granny gears. I have 6 I can use down there (not cross chaining) and most of the time it’s not the 24 36 combo I need. Like someone mentioned above I try and get more in the middle of the cassette before dropping on the granny and then if anything when I hit the hill I can start shifting down in the back as the hill gets worse. If you go all the way low (biggest) on the cassette and then drop on that small chain ring it just seems harder knowing where I’m at with the gears.

I’ll take all the help I can get with gears, and I don’t see why your bike shop had any issues with making that little change.