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 $153.33   8.651%   $166.5946   $166.59   $166.60   $166.60  
  Purchase #2 $53.56   6.254%   $56.9096   $56.91   $56.90   $56.90  
  Purchase #3 $16.75   4.673%   $17.5327   $17.53   $17.55   $17.50  
  Purchase #4 $182.68   4.374%   $190.6704   $190.67   $190.65   $190.70  
  Purchase #5 $91.65   6.471%   $97.5807   $97.58   $97.60   $97.60  
  Purchase #6 $83.67   3.662%   $86.734   $86.73   $86.75   $86.70  
  Purchase #7 $39.64   5.214%   $41.7068   $41.71   $41.70   $41.70  
  7 purchase totals = $621.28       $657.7288   $657.72   $657.75   $657.70