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 $234.38   6.517%   $249.6545   $249.65   $249.65   $249.70  
  Purchase #2 $112.28   11.289%   $124.9553   $124.96   $124.95   $125.00  
  Purchase #3 $86.86   4.458%   $90.7322   $90.73   $90.75   $90.70  
  Purchase #4 $145.81   9.513%   $159.6809   $159.68   $159.70   $159.70  
  Purchase #5 $169.37   8.344%   $183.5022   $183.50   $183.50   $183.50  
  Purchase #6 $232.85   9.635%   $255.2851   $255.29   $255.30   $255.30  
  Purchase #7 $100.62   12.713%   $113.4118   $113.41   $113.40   $113.40  
  7 purchase totals = $1,082.17       $1,177.2220   $1,177.22   $1,177.25   $1,177.30