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 $17.61   6.524%   $18.7589   $18.76   $18.75   $18.80  
  Purchase #2 $83.98   9.117%   $91.6365   $91.64   $91.65   $91.60  
  Purchase #3 $212.33   11.768%   $237.317   $237.32   $237.30   $237.30  
  Purchase #4 $73.62   10.269%   $81.18   $81.18   $81.20   $81.20  
  Purchase #5 $227.86   12.350%   $256.0007   $256.00   $256.00   $256.00  
  Purchase #6 $52.54   5.162%   $55.2521   $55.25   $55.25   $55.30  
  Purchase #7 $233.78   11.287%   $260.1667   $260.17   $260.15   $260.20  
  7 purchase totals = $901.72       $1,000.3119   $1,000.32   $1,000.30   $1,000.40