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 $8.32   9.597%   $9.1185   $9.12   $9.10   $9.10  
  Purchase #2 $137.93   6.201%   $146.483   $146.48   $146.50   $146.50  
  Purchase #3 $18.64   10.374%   $20.5737   $20.57   $20.55   $20.60  
  Purchase #4 $231.70   10.561%   $256.1698   $256.17   $256.15   $256.20  
  Purchase #5 $206.42   4.733%   $216.1899   $216.19   $216.20   $216.20  
  Purchase #6 $188.73   3.789%   $195.881   $195.88   $195.90   $195.90  
  Purchase #7 $192.56   6.501%   $205.0783   $205.08   $205.10   $205.10  
  7 purchase totals = $984.30       $1,049.4942   $1,049.49   $1,049.50   $1,049.60