temperature fluctuation during fermentation

I have 5 gallons of AHS California Common Ale in primary and it is bubbling away. I used the recommended yeast, White Labs WL810 and I have a question or two.White Labs calls this a lager yeast but the temperature range is pretty wide 58° to 65° with a note saying it can go down to 50°. The instructions from AHS said to pitch at 70° which I did and held it there until fermentation started - about 12 hours. I then added an ice bottle to my cooler that the carboy sits in and dropped the temp down to about 58°.

With the cooler weather and the ice bottles I can keep things pretty cool but I am wondering about temperature fluctuation. I listened to a Brew Strong Pod Cast on fermentation and they say you need to hold the temp within 1 or 2 degrees or you can stress out the yeast. I'm probably getting more like 5 or 6 (58° - 63°) degrees of variation with my ice bottle technique.

Is precise temp control during fermentation that critical? I thought as long as I kept it towards the low end of the yeast's range I was OK. Not so???

I've made hundreds of good beers without carefully controlling the fermentation temperatures, but I've also made hundreds of beers that could have been better.
I now carefully control my fermentation temperatures, and have found that the ratio of good to not so good beers has increased dramatically.

-a.

It is ideal to keep your temps as constant as possible, but a lot of times it's easier said than done without a good fermentation chamber. As long as you stay in the given range you should be ok, but most importantly don't let it get too hot. That's much worse than a little too cool, IMO. I'm willing to bet the actual temps inside your fermentor are more stable than you think.