Not ideal means that, yeah, the tube is stretched thinner and yes that can cause more flats. This is an argument bordering on the theoretical.If you get a small piece of glass that nicks your tube it is probably going to flat sooner or later anyway. If you give me the option of going to the store and buying a new tube vs using a road tube in a cross tire, I will put the road tube in the cross tire every time and NOT think about it again. If I get a flat, well darn I will change it. But the question was not WHAT is the best tube to use with my 35mm tires, it was can I use thinner tube in a fatter tire. The answer is Yeah, no problem, use it. This happens all the time.
You go to a shop, they don't have the right tube in stock, what do you do? Ride around looking for another shop? Mail order the right tubes? What do you do in the meantime. You use the tube that gets the job done. What do you think people did before 29'er tubes were readily available? They ran cross tubes "rated" for 35mm in 50mm plus tires. Hell you can run 29'er tires on open pro road rims and road tubes. If you have the choice, get wide rims and wide tires, if you want to try it out with what you got, do it.
This is not new, this is not a big deal. If you give me the choice to run the right tubes, I will. When you order new tires, go ahead and get the right tubes... But 10 years ago, it was not that easy to find 35mm presta tubes in a shop. You could easily get hybrid shrader tubes in fat sizes, and skinny presta tubes. Everyone ran presta road tubes in there cross tires. And this is not just for racing. I commuted on cross tires for years.
What do you do Peter if someone comes into the shop looking for a 35mm presta tube and all you have are 25mm tubes left in stock? Do you send them away with no tubes because it is a very bad idea? What if they are tourists coming through at closing time? I am not telling you to sell people the wrong size tube.
The argument that because I race, I don't experience this, or that since I race I am not an every day rider that needs reliable wheels is just plain old ignorant. What is the mental picture here? That I ride on the trainer all year, except for the weekly hammerhead ride where I cleverly put new tires and tubes in each time to avoid flats? I don't have a follow car when I train, ride or commute and I commute every day. I don't have a mechanic who is giving me fresh tubes for every rides. I maintain my own bikes, I have worked in shops, I have worked for manufacturers and done race support, I ride some of my bikes into the ground, and I ride some of them ocassionally with long pauses between. I have a fair number of bikes and ride them alot and very differently.
If you have consistent problems pulling the valve out of the tube, work really hard on your pumping technique. Thats operator error or shitty quality innertubes. If you are buying tubes for 1.50 a tube from bikes direct or some other mail order house, maybe you need to spend a few more dollars on innertubes Worry about tube matching if you are about to leave for a long tour or you get new tires or you have the option sitting there in front of you. If you are in a pinch, run what you got. Not a bad idea. Not lucky to get away with it.
I don't even know where to begin with Landotters argument that you should run 35mm rated tubes in 32mm tires. All I know is that I am very impressed you have a LBS that can afford to stock tubes in 3mm increments.
Do whatever you wish, if you have good flat juju, keep on doing what you are doing, that is a hard thing to maintain. I am not telling you NOT to run the correct sized tubes. In fact I love the right sized tubes. And I like nice tubes... But please don't tell people that it is a "very bad idea" to run undersized tubes. That is hyperbole that serves no purpose.