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 $29.90   6.481%   $31.8378   $31.84   $31.85   $31.80  
  Purchase #2 $117.46   4.730%   $123.0159   $123.02   $123.00   $123.00  
  Purchase #3 $210.87   13.488%   $239.3121   $239.31   $239.30   $239.30  
  Purchase #4 $126.71   10.276%   $139.7307   $139.73   $139.75   $139.70  
  Purchase #5 $9.48   11.187%   $10.5405   $10.54   $10.55   $10.50  
  Purchase #6 $73.79   7.822%   $79.5619   $79.56   $79.55   $79.60  
  Purchase #7 $26.25   7.506%   $28.2203   $28.22   $28.20   $28.20  
  7 purchase totals = $594.46       $652.2192   $652.22   $652.20   $652.10