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 $170.84   8.438%   $185.2555   $185.26   $185.25   $185.30  
  Purchase #2 $239.62   8.243%   $259.3719   $259.37   $259.35   $259.40  
  Purchase #3 $172.60   8.142%   $186.6531   $186.65   $186.65   $186.70  
  Purchase #4 $213.57   7.623%   $229.8504   $229.85   $229.85   $229.90  
  Purchase #5 $130.40   8.128%   $140.9989   $141.00   $141.00   $141.00  
  Purchase #6 $180.90   11.632%   $201.9423   $201.94   $201.95   $201.90  
  Purchase #7 $250.48   6.266%   $266.1751   $266.18   $266.20   $266.20  
  7 purchase totals = $1,358.41       $1,470.2472   $1,470.25   $1,470.25   $1,470.40