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 $39.97   3.637%   $41.4237   $41.42   $41.40   $41.40  
  Purchase #2 $160.30   6.300%   $170.3989   $170.40   $170.40   $170.40  
  Purchase #3 $91.45   6.310%   $97.2205   $97.22   $97.20   $97.20  
  Purchase #4 $64.32   7.304%   $69.0179   $69.02   $69.00   $69.00  
  Purchase #5 $105.61   10.537%   $116.7381   $116.74   $116.75   $116.70  
  Purchase #6 $22.76   3.489%   $23.5541   $23.55   $23.55   $23.60  
  Purchase #7 $123.25   11.740%   $137.7196   $137.72   $137.70   $137.70  
  7 purchase totals = $607.66       $656.0728   $656.07   $656.00   $656.00