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 $218.58   11.252%   $243.1746   $243.17   $243.15   $243.20  
  Purchase #2 $95.90   9.423%   $104.9367   $104.94   $104.95   $104.90  
  Purchase #3 $30.81   3.592%   $31.9167   $31.92   $31.90   $31.90  
  Purchase #4 $249.72   7.747%   $269.0658   $269.07   $269.05   $269.10  
  Purchase #5 $16.50   12.573%   $18.5745   $18.57   $18.55   $18.60  
  Purchase #6 $32.60   3.552%   $33.758   $33.76   $33.75   $33.80  
  Purchase #7 $247.45   9.691%   $271.4304   $271.43   $271.45   $271.40  
  7 purchase totals = $891.56       $972.8567   $972.86   $972.80   $972.90