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 $12.48   4.118%   $12.9939   $12.99   $13.00   $13.00  
  Purchase #2 $26.36   9.759%   $28.9325   $28.93   $28.95   $28.90  
  Purchase #3 $242.38   5.749%   $256.3144   $256.31   $256.30   $256.30  
  Purchase #4 $142.52   4.710%   $149.2327   $149.23   $149.25   $149.20  
  Purchase #5 $192.53   3.775%   $199.798   $199.80   $199.80   $199.80  
  Purchase #6 $63.91   5.832%   $67.6372   $67.64   $67.65   $67.60  
  Purchase #7 $5.71   9.752%   $6.2668   $6.27   $6.25   $6.30  
  7 purchase totals = $685.89       $721.1755   $721.17   $721.20   $721.10