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 $246.35   8.114%   $266.3388   $266.34   $266.35   $266.30  
  Purchase #2 $168.79   4.395%   $176.2083   $176.21   $176.20   $176.20  
  Purchase #3 $57.55   12.337%   $64.6499   $64.65   $64.65   $64.60  
  Purchase #4 $150.68   7.200%   $161.529   $161.53   $161.55   $161.50  
  Purchase #5 $7.88   9.277%   $8.611   $8.61   $8.60   $8.60  
  Purchase #6 $39.73   11.547%   $44.3176   $44.32   $44.30   $44.30  
  Purchase #7 $66.79   13.387%   $75.7312   $75.73   $75.75   $75.70  
  7 purchase totals = $737.77       $797.3858   $797.39   $797.40   $797.20