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 $18.49   12.555%   $20.8114   $20.81   $20.80   $20.80  
  Purchase #2 $179.72   5.335%   $189.3081   $189.31   $189.30   $189.30  
  Purchase #3 $231.49   12.264%   $259.8799   $259.88   $259.90   $259.90  
  Purchase #4 $72.76   6.416%   $77.4283   $77.43   $77.45   $77.40  
  Purchase #5 $25.66   13.216%   $29.0512   $29.05   $29.05   $29.10  
  Purchase #6 $91.30   13.745%   $103.8492   $103.85   $103.85   $103.80  
  Purchase #7 $18.37   13.264%   $20.8066   $20.81   $20.80   $20.80  
  7 purchase totals = $637.79       $701.1347   $701.14   $701.15   $701.10