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 $249.65   12.210%   $280.1323   $280.13   $280.15   $280.10  
  Purchase #2 $219.38   12.115%   $245.9579   $245.96   $245.95   $246.00  
  Purchase #3 $202.85   13.167%   $229.5593   $229.56   $229.55   $229.60  
  Purchase #4 $87.26   5.411%   $91.9816   $91.98   $92.00   $92.00  
  Purchase #5 $139.37   4.224%   $145.257   $145.26   $145.25   $145.30  
  Purchase #6 $213.55   4.186%   $222.4892   $222.49   $222.50   $222.50  
  Purchase #7 $79.69   8.857%   $86.7481   $86.75   $86.75   $86.70  
  7 purchase totals = $1,191.75       $1,302.1254   $1,302.13   $1,302.15   $1,302.20