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 $7.99   11.190%   $8.8841   $8.88   $8.90   $8.90  
  Purchase #2 $15.53   13.149%   $17.572   $17.57   $17.55   $17.60  
  Purchase #3 $171.76   13.611%   $195.1383   $195.14   $195.15   $195.10  
  Purchase #4 $165.83   11.157%   $184.3317   $184.33   $184.35   $184.30  
  Purchase #5 $20.40   4.446%   $21.307   $21.31   $21.30   $21.30  
  Purchase #6 $49.74   7.755%   $53.5973   $53.60   $53.60   $53.60  
  Purchase #7 $71.65   4.115%   $74.5984   $74.60   $74.60   $74.60  
  7 purchase totals = $502.90       $555.4288   $555.43   $555.45   $555.40