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 $164.59   9.357%   $179.9907   $179.99   $180.00   $180.00  
  Purchase #2 $121.40   13.600%   $137.9104   $137.91   $137.90   $137.90  
  Purchase #3 $184.76   10.557%   $204.2651   $204.27   $204.25   $204.30  
  Purchase #4 $112.91   9.666%   $123.8239   $123.82   $123.80   $123.80  
  Purchase #5 $86.90   3.632%   $90.0562   $90.06   $90.05   $90.10  
  Purchase #6 $232.65   8.812%   $253.1511   $253.15   $253.15   $253.20  
  Purchase #7 $29.47   4.806%   $30.8863   $30.89   $30.90   $30.90  
  7 purchase totals = $932.68       $1,020.0837   $1,020.09   $1,020.05   $1,020.20