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 $173.32   4.351%   $180.8612   $180.86   $180.85   $180.90  
  Purchase #2 $58.80   3.806%   $61.0379   $61.04   $61.05   $61.00  
  Purchase #3 $38.53   9.168%   $42.0624   $42.06   $42.05   $42.10  
  Purchase #4 $215.25   12.327%   $241.7839   $241.78   $241.80   $241.80  
  Purchase #5 $5.68   12.537%   $6.3921   $6.39   $6.40   $6.40  
  Purchase #6 $89.44   9.800%   $98.2051   $98.21   $98.20   $98.20  
  Purchase #7 $35.82   12.703%   $40.3702   $40.37   $40.35   $40.40  
  7 purchase totals = $616.84       $670.7128   $670.71   $670.70   $670.80