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 $170.82   5.212%   $179.7231   $179.72   $179.70   $179.70  
  Purchase #2 $227.76   9.831%   $250.1511   $250.15   $250.15   $250.20  
  Purchase #3 $241.27   9.781%   $264.8686   $264.87   $264.85   $264.90  
  Purchase #4 $220.91   10.700%   $244.5474   $244.55   $244.55   $244.50  
  Purchase #5 $24.37   4.598%   $25.4905   $25.49   $25.50   $25.50  
  Purchase #6 $62.54   10.283%   $68.971   $68.97   $68.95   $69.00  
  Purchase #7 $58.82   11.706%   $65.7055   $65.71   $65.70   $65.70  
  7 purchase totals = $1,006.49       $1,099.4572   $1,099.46   $1,099.40   $1,099.50