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 $201.80   8.182%   $218.3113   $218.31   $218.30   $218.30  
  Purchase #2 $55.61   7.216%   $59.6228   $59.62   $59.60   $59.60  
  Purchase #3 $13.92   9.624%   $15.2597   $15.26   $15.25   $15.30  
  Purchase #4 $56.97   7.284%   $61.1197   $61.12   $61.10   $61.10  
  Purchase #5 $49.78   11.159%   $55.335   $55.34   $55.35   $55.30  
  Purchase #6 $231.74   7.874%   $249.9872   $249.99   $250.00   $250.00  
  Purchase #7 $76.43   7.222%   $81.9498   $81.95   $81.95   $81.90  
  7 purchase totals = $686.25       $741.5855   $741.59   $741.55   $741.50