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 $18.44   8.872%   $20.076   $20.08   $20.10   $20.10  
  Purchase #2 $200.55   6.815%   $214.2175   $214.22   $214.20   $214.20  
  Purchase #3 $176.43   7.529%   $189.7134   $189.71   $189.70   $189.70  
  Purchase #4 $163.26   4.825%   $171.1373   $171.14   $171.15   $171.10  
  Purchase #5 $39.42   8.564%   $42.7959   $42.80   $42.80   $42.80  
  Purchase #6 $22.27   8.545%   $24.173   $24.17   $24.15   $24.20  
  Purchase #7 $25.27   12.302%   $28.3787   $28.38   $28.40   $28.40  
  7 purchase totals = $645.64       $690.4918   $690.50   $690.50   $690.50