Excel – How to prevent automatic rounding of numbers in excel 2010

You could wrap your complex formula's in a UDF that does the formatting:

Function FormatComplex(r As Variant, fmt As String) As String
    With Application.WorksheetFunction
        FormatComplex = Format(.ImReal(r), fmt) & Format(.Imaginary(r), fmt) & "i"
    End With
End Function

Call it like

=FormatComplex(IMSQRT(-2),"+0.000")

result:

+0.000+1.414i

You don't have to change the cells' formatting one at a time. You can apply the same formats to whole columns, ranges or even sheets. If you know that you'll have number in all the cells, make sure you set them to one of the number formats. The General formatting option is treated by Excel as a sort of "no formatting" option, so it will often try to guess what specific format you want it in.

Alternatively, you may want to look at this answer and see if it applies to your situation:

Excel assumes you have not applied a format if you use general, so it tries to find the best format for the data you entered. To disable this, go to File => Options => Advanced and uncheck the option for "Extend data range formats and formulas."