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 $19.89   7.523%   $21.3863   $21.39   $21.40   $21.40  
  Purchase #2 $7.44   10.697%   $8.2359   $8.24   $8.25   $8.20  
  Purchase #3 $221.90   11.483%   $247.3808   $247.38   $247.40   $247.40  
  Purchase #4 $120.63   6.865%   $128.9112   $128.91   $128.90   $128.90  
  Purchase #5 $45.64   12.571%   $51.3774   $51.38   $51.40   $51.40  
  Purchase #6 $57.63   10.471%   $63.6644   $63.66   $63.65   $63.70  
  Purchase #7 $163.99   8.568%   $178.0407   $178.04   $178.05   $178.00  
  7 purchase totals = $637.12       $698.9967   $699.00   $699.05   $699.00