If you are willing to use extra columns and use "Precision as displayed" option, it could be done very simply as follows:
First, check "Precision as diplayed" in Tools/Options/Calculations
Col A: Using "General" number format, input orginal value
Col B: Set column number format to scientific, with the desired precision, and have the number from column A
Col C: Set column number format to "General" and set it equal to the value in column B
For some examples assuming "decimal places" for scientific format set at 3:
Col A: 156.7811
Col B: 1.568 E 2
Col C: 156.8
Col A: -0.00125668
Col B: - 1.257 E -3
Col C: -0.001257
Of course, it may be cleaner if you hide column B (and possibly column A as well) when you are done...
-- MechEng2005