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 $137.55   5.715%   $145.411   $145.41   $145.40   $145.40  
  Purchase #2 $57.57   11.566%   $64.2285   $64.23   $64.25   $64.20  
  Purchase #3 $6.59   7.335%   $7.0734   $7.07   $7.05   $7.10  
  Purchase #4 $217.79   3.574%   $225.5738   $225.57   $225.55   $225.60  
  Purchase #5 $204.36   7.286%   $219.2497   $219.25   $219.25   $219.20  
  Purchase #6 $75.92   10.481%   $83.8772   $83.88   $83.90   $83.90  
  Purchase #7 $133.84   8.482%   $145.1923   $145.19   $145.20   $145.20  
  7 purchase totals = $833.62       $890.6059   $890.60   $890.60   $890.60