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 $12.77   8.816%   $13.8958   $13.90   $13.90   $13.90  
  Purchase #2 $35.78   11.778%   $39.9942   $39.99   $40.00   $40.00  
  Purchase #3 $61.47   6.167%   $65.2609   $65.26   $65.25   $65.30  
  Purchase #4 $136.55   7.567%   $146.8827   $146.88   $146.90   $146.90  
  Purchase #5 $100.80   7.589%   $108.4497   $108.45   $108.45   $108.40  
  Purchase #6 $5.45   13.640%   $6.1934   $6.19   $6.20   $6.20  
  Purchase #7 $237.48   11.116%   $263.8783   $263.88   $263.90   $263.90  
  7 purchase totals = $590.30       $644.5550   $644.55   $644.60   $644.60