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 $35.44   10.515%   $39.1665   $39.17   $39.15   $39.20  
  Purchase #2 $226.77   6.245%   $240.9318   $240.93   $240.95   $240.90  
  Purchase #3 $208.73   7.647%   $224.6916   $224.69   $224.70   $224.70  
  Purchase #4 $107.76   10.457%   $119.0285   $119.03   $119.05   $119.00  
  Purchase #5 $95.68   10.502%   $105.7283   $105.73   $105.75   $105.70  
  Purchase #6 $65.75   12.624%   $74.0503   $74.05   $74.05   $74.10  
  Purchase #7 $214.82   13.678%   $244.2031   $244.20   $244.20   $244.20  
  7 purchase totals = $954.95       $1,047.8001   $1,047.80   $1,047.85   $1,047.80