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 $5.42   8.618%   $5.8871   $5.89   $5.90   $5.90  
  Purchase #2 $81.91   5.793%   $86.655   $86.66   $86.65   $86.70  
  Purchase #3 $237.56   9.454%   $260.0189   $260.02   $260.00   $260.00  
  Purchase #4 $5.45   7.393%   $5.8529   $5.85   $5.85   $5.90  
  Purchase #5 $151.81   11.370%   $169.0708   $169.07   $169.05   $169.10  
  Purchase #6 $58.84   7.166%   $63.0565   $63.06   $63.05   $63.10  
  Purchase #7 $52.39   7.662%   $56.4041   $56.40   $56.40   $56.40  
  7 purchase totals = $593.38       $646.9453   $646.95   $646.90   $647.10