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 $125.93   9.142%   $137.4425   $137.44   $137.45   $137.40  
  Purchase #2 $227.25   6.652%   $242.3667   $242.37   $242.35   $242.40  
  Purchase #3 $222.57   7.591%   $239.4653   $239.47   $239.45   $239.50  
  Purchase #4 $151.74   4.533%   $158.6184   $158.62   $158.60   $158.60  
  Purchase #5 $46.66   12.469%   $52.478   $52.48   $52.50   $52.50  
  Purchase #6 $90.58   5.417%   $95.4867   $95.49   $95.50   $95.50  
  Purchase #7 $43.78   11.134%   $48.6545   $48.65   $48.65   $48.70  
  7 purchase totals = $908.51       $974.5121   $974.52   $974.50   $974.60