I agree with your LBS on going with the larger tires, but not sure by what he meant by underinflate. Take the guess work and opinions out of the equation and use this calculator instead: dorkypantsr.usbike tire pressure calculator.htmlOpen ↗ Use the second calculator and enter your fully dressed ready to ride body weight plus your fully equipped ready to ride bicycle weight, enter your tire size front and rear and read your recommended PSI. The only exclusion to this is if the tire package has their own recommended weight/psi scale then use that scale instead.
As far as tires go, if the inflation is correct you shouldn't get any snake bike flats regardless of the tire. If you're getting flats from road debris then you need a better tire at warding off flats. There are kinds of good highly flat resistant tires on the market today, Vittoria Rubino Pro Slick, Hutchinson Intensive Longwearing (not the tubeless version), Specialized Roubaix Pro, Specialized All Condition Armadillo, Michelin Pro 4 Endurance, Maxxis Re-Fuseor the Continental Gator Hardshell (their other tires suck, they use ultra thin sidewalls that are very susceptible to damage). Some of these tires are very expensive but if wait for sales and end of season closeouts you can get really good deals from Performance Bike and Nashbar, and on Amazon. Maxxis Re Fuse is the best buy tire when no tires are on sale. I never pay more than $32 for a tire and I end up with $60 and more retail tires.
Most tire liners suck. The plastic strip kind like the Mr Tuffy and Slime don't work that great plus there is quite a hassle to make sure the liner stays in place after the tube is installed and you start to air up the tire. I found one liner that works superbly, and that's the Panaracer FlatAway, however it's a one time use liner and it cost about $15 each, but their a lot tougher than a Mr Tuffy, how tough you ask? This tough. I took this tack: lowes.compd 58218 37672 532362Open ↗ And tried to push it through the FlatAway liner, when that failed I put my thumb (like the idiot that I am) over the tack to exert more pressure and still failed to penetrate the liner, but the tack went right through a piece of Mr Tuffy I had with little difficulty. Next I discovered while sizing the liner for the tire I was have a horrible time trying to cut the liner with my scissors and my hand was even hurting, but those same scissors cut the Mr Tuffy like butter. The Panaracer FlatAway liner has a sticky backing, you peel off the paper as you lay the liner inside the tire, the sticky side will stick to the tire and will not come out of place as you put the tube in. Also due to the kevlar cloth construction of the FlatAway liner they claim snakebite flats are reduced vs plastic liners, and they only weigh 32 grams vs 98 for Mt Tuffy. So if you decide on using a liner skip the nylon plastic [censored] and use the Panaracer FlatAway. I use them but only on my rear tires because that's where most of the flats occur anyways and don't want to waste money on the front.
Slime tubes in road bikes don't work, once the psi exceeds 70 the [censored] just blows out of the smallest hole, and their presta valves are cheapest crappiest valves I ever used. Thorn tubes offer some limited protection but add a lot of weight, Your first line of defense is the tire, then a liner, if something makes it past those two a tube won't stop it so just use a regular tube.
