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 $246.94   5.304%   $260.0377   $260.04   $260.05   $260.00  
  Purchase #2 $165.35   9.688%   $181.3691   $181.37   $181.35   $181.40  
  Purchase #3 $141.76   3.483%   $146.6975   $146.70   $146.70   $146.70  
  Purchase #4 $12.91   7.438%   $13.8702   $13.87   $13.85   $13.90  
  Purchase #5 $54.56   9.261%   $59.6128   $59.61   $59.60   $59.60  
  Purchase #6 $137.98   6.468%   $146.9045   $146.90   $146.90   $146.90  
  Purchase #7 $163.79   9.239%   $178.9226   $178.92   $178.90   $178.90  
  7 purchase totals = $923.29       $987.4144   $987.41   $987.35   $987.40