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 $74.33   12.598%   $83.6941   $83.69   $83.70   $83.70  
  Purchase #2 $224.96   9.432%   $246.1782   $246.18   $246.20   $246.20  
  Purchase #3 $216.94   11.502%   $241.8924   $241.89   $241.90   $241.90  
  Purchase #4 $225.58   11.754%   $252.0947   $252.09   $252.10   $252.10  
  Purchase #5 $27.69   10.773%   $30.673   $30.67   $30.65   $30.70  
  Purchase #6 $86.99   9.785%   $95.502   $95.50   $95.50   $95.50  
  Purchase #7 $28.99   6.518%   $30.8796   $30.88   $30.90   $30.90  
  7 purchase totals = $885.48       $980.9140   $980.90   $980.95   $981.00