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 $246.62   8.437%   $267.4273   $267.43   $267.45   $267.40  
  Purchase #2 $146.74   9.557%   $160.7639   $160.76   $160.75   $160.80  
  Purchase #3 $246.27   7.167%   $263.9202   $263.92   $263.90   $263.90  
  Purchase #4 $245.59   3.845%   $255.0329   $255.03   $255.05   $255.00  
  Purchase #5 $44.30   9.746%   $48.6175   $48.62   $48.60   $48.60  
  Purchase #6 $43.61   7.686%   $46.9619   $46.96   $46.95   $47.00  
  Purchase #7 $235.44   6.626%   $251.0403   $251.04   $251.05   $251.00  
  7 purchase totals = $1,208.57       $1,293.7640   $1,293.76   $1,293.75   $1,293.70