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 $241.29   3.280%   $249.2043   $249.20   $249.20   $249.20  
  Purchase #2 $55.56   7.571%   $59.7664   $59.77   $59.75   $59.80  
  Purchase #3 $13.46   6.831%   $14.3795   $14.38   $14.40   $14.40  
  Purchase #4 $65.49   4.523%   $68.4521   $68.45   $68.45   $68.50  
  Purchase #5 $106.61   3.684%   $110.5375   $110.54   $110.55   $110.50  
  Purchase #6 $241.39   5.316%   $254.2223   $254.22   $254.20   $254.20  
  Purchase #7 $27.33   3.283%   $28.2272   $28.23   $28.25   $28.20  
  7 purchase totals = $751.13       $784.7893   $784.79   $784.80   $784.80