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 $13.29   3.840%   $13.8003   $13.80   $13.80   $13.80  
  Purchase #2 $56.79   5.180%   $59.7317   $59.73   $59.75   $59.70  
  Purchase #3 $31.28   4.568%   $32.7089   $32.71   $32.70   $32.70  
  Purchase #4 $135.38   11.873%   $151.4537   $151.45   $151.45   $151.50  
  Purchase #5 $116.94   4.443%   $122.1356   $122.14   $122.15   $122.10  
  Purchase #6 $208.64   6.785%   $222.7962   $222.80   $222.80   $222.80  
  Purchase #7 $79.43   6.467%   $84.5667   $84.57   $84.55   $84.60  
  7 purchase totals = $641.75       $687.1931   $687.20   $687.20   $687.20