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 $55.64   13.152%   $62.9578   $62.96   $62.95   $63.00  
  Purchase #2 $19.72   6.426%   $20.9872   $20.99   $21.00   $21.00  
  Purchase #3 $98.88   10.179%   $108.945   $108.95   $108.95   $108.90  
  Purchase #4 $216.26   6.511%   $230.3407   $230.34   $230.35   $230.30  
  Purchase #5 $26.48   9.589%   $29.0192   $29.02   $29.00   $29.00  
  Purchase #6 $58.40   7.382%   $62.7111   $62.71   $62.70   $62.70  
  Purchase #7 $79.59   6.791%   $84.995   $85.00   $85.00   $85.00  
  7 purchase totals = $554.97       $599.9560   $599.97   $599.95   $599.90