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 $16.99   8.770%   $18.48   $18.48   $18.50   $18.50  
  Purchase #2 $124.96   10.489%   $138.0671   $138.07   $138.05   $138.10  
  Purchase #3 $55.72   12.848%   $62.8789   $62.88   $62.90   $62.90  
  Purchase #4 $120.67   7.722%   $129.9881   $129.99   $130.00   $130.00  
  Purchase #5 $249.41   10.279%   $275.0469   $275.05   $275.05   $275.00  
  Purchase #6 $78.96   12.754%   $89.0306   $89.03   $89.05   $89.00  
  Purchase #7 $214.74   12.158%   $240.8481   $240.85   $240.85   $240.80  
  7 purchase totals = $861.45       $954.3397   $954.35   $954.40   $954.30