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 $19.64   8.361%   $21.2821   $21.28   $21.30   $21.30  
  Purchase #2 $11.74   11.313%   $13.0681   $13.07   $13.05   $13.10  
  Purchase #3 $83.27   10.489%   $92.0042   $92.00   $92.00   $92.00  
  Purchase #4 $208.91   9.409%   $228.5663   $228.57   $228.55   $228.60  
  Purchase #5 $12.99   7.180%   $13.9227   $13.92   $13.90   $13.90  
  Purchase #6 $179.90   10.568%   $198.9118   $198.91   $198.90   $198.90  
  Purchase #7 $107.39   4.849%   $112.5973   $112.60   $112.60   $112.60  
  7 purchase totals = $623.84       $680.3525   $680.35   $680.30   $680.40