Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $9.99   9.746%   $10.9636   $10.96   $10.95   $11.00  
  Purchase #2 $209.72   8.756%   $228.0831   $228.08   $228.10   $228.10  
  Purchase #3 $147.60   8.383%   $159.9733   $159.97   $159.95   $160.00  
  Purchase #4 $145.37   10.176%   $160.1629   $160.16   $160.15   $160.20  
  Purchase #5 $136.92   3.185%   $141.2809   $141.28   $141.30   $141.30  
  Purchase #6 $72.79   5.498%   $76.792   $76.79   $76.80   $76.80  
  Purchase #7 $48.61   7.342%   $52.1789   $52.18   $52.20   $52.20  
  7 purchase totals = $771.00       $829.4347   $829.42   $829.45   $829.60