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 $12.30   3.874%   $12.7765   $12.78   $12.80   $12.80  
  Purchase #2 $203.53   7.176%   $218.1353   $218.14   $218.15   $218.10  
  Purchase #3 $99.81   6.590%   $106.3875   $106.39   $106.40   $106.40  
  Purchase #4 $207.85   7.231%   $222.8796   $222.88   $222.90   $222.90  
  Purchase #5 $184.73   10.542%   $204.2042   $204.20   $204.20   $204.20  
  Purchase #6 $15.37   4.534%   $16.0669   $16.07   $16.05   $16.10  
  Purchase #7 $196.55   13.824%   $223.7211   $223.72   $223.70   $223.70  
  7 purchase totals = $920.14       $1,004.1711   $1,004.18   $1,004.20   $1,004.20