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 $30.69   8.441%   $33.2805   $33.28   $33.30   $33.30  
  Purchase #2 $98.32   11.735%   $109.8579   $109.86   $109.85   $109.90  
  Purchase #3 $122.47   5.668%   $129.4116   $129.41   $129.40   $129.40  
  Purchase #4 $12.64   8.446%   $13.7076   $13.71   $13.70   $13.70  
  Purchase #5 $14.93   7.578%   $16.0614   $16.06   $16.05   $16.10  
  Purchase #6 $81.27   9.475%   $88.9703   $88.97   $88.95   $89.00  
  Purchase #7 $76.35   13.173%   $86.4076   $86.41   $86.40   $86.40  
  7 purchase totals = $436.67       $477.6969   $477.70   $477.65   $477.80