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 $19.25   9.241%   $21.0289   $21.03   $21.05   $21.00  
  Purchase #2 $12.95   6.572%   $13.8011   $13.80   $13.80   $13.80  
  Purchase #3 $183.70   11.654%   $205.1084   $205.11   $205.10   $205.10  
  Purchase #4 $235.90   4.523%   $246.5698   $246.57   $246.55   $246.60  
  Purchase #5 $109.68   6.625%   $116.9463   $116.95   $116.95   $116.90  
  Purchase #6 $24.88   7.302%   $26.6967   $26.70   $26.70   $26.70  
  Purchase #7 $225.70   9.371%   $246.8503   $246.85   $246.85   $246.90  
  7 purchase totals = $812.06       $877.0015   $877.01   $877.00   $877.00