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 $249.74   13.639%   $283.802   $283.80   $283.80   $283.80  
  Purchase #2 $136.37   7.778%   $146.9769   $146.98   $147.00   $147.00  
  Purchase #3 $239.47   4.465%   $250.1623   $250.16   $250.15   $250.20  
  Purchase #4 $144.72   13.775%   $164.6552   $164.66   $164.65   $164.70  
  Purchase #5 $44.63   13.452%   $50.6336   $50.63   $50.65   $50.60  
  Purchase #6 $118.75   12.213%   $133.2529   $133.25   $133.25   $133.30  
  Purchase #7 $62.31   7.811%   $67.177   $67.18   $67.20   $67.20  
  7 purchase totals = $995.99       $1,096.6599   $1,096.66   $1,096.70   $1,096.80