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 $51.34   4.389%   $53.5933   $53.59   $53.60   $53.60  
  Purchase #2 $212.51   9.487%   $232.6708   $232.67   $232.65   $232.70  
  Purchase #3 $194.73   12.535%   $219.1394   $219.14   $219.15   $219.10  
  Purchase #4 $109.64   5.799%   $115.998   $116.00   $116.00   $116.00  
  Purchase #5 $218.40   3.218%   $225.4281   $225.43   $225.45   $225.40  
  Purchase #6 $11.52   4.671%   $12.0581   $12.06   $12.05   $12.10  
  Purchase #7 $130.33   10.644%   $144.2023   $144.20   $144.20   $144.20  
  7 purchase totals = $928.47       $1,003.0900   $1,003.09   $1,003.10   $1,003.10