The term significant DOES NOT mean "mathematically important." Certainly, the zero at the end of 350 is mathematically important. However, it is NOT significant. Remember, the term significant figure simply means that the figure was MEASURED. When you write a number as 350, you are telling scientists that the measuring device you used was not accurate enough to read the ones place. Thus, the ones place was not measured and the zero there is not significant. If you had a measuring device that was sensitive to the ones place, and if that device indicated that a zero belongs in the ones place, then you would have to report the answer as 3.50 x10^2. That tells the reader that the zero in the ones place was measured and is therefore significant.