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 $250.69   13.188%   $283.751   $283.75   $283.75   $283.80  
  Purchase #2 $246.97   5.126%   $259.6297   $259.63   $259.65   $259.60  
  Purchase #3 $101.64   3.220%   $104.9128   $104.91   $104.90   $104.90  
  Purchase #4 $62.72   11.298%   $69.8061   $69.81   $69.80   $69.80  
  Purchase #5 $102.74   10.631%   $113.6623   $113.66   $113.65   $113.70  
  Purchase #6 $179.28   8.749%   $194.9652   $194.97   $194.95   $195.00  
  Purchase #7 $13.63   11.293%   $15.1692   $15.17   $15.15   $15.20  
  7 purchase totals = $957.67       $1,041.8963   $1,041.90   $1,041.85   $1,042.00