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 $242.76   11.838%   $271.4979   $271.50   $271.50   $271.50  
  Purchase #2 $233.68   6.439%   $248.7267   $248.73   $248.75   $248.70  
  Purchase #3 $63.91   13.586%   $72.5928   $72.59   $72.60   $72.60  
  Purchase #4 $194.68   4.143%   $202.7456   $202.75   $202.75   $202.70  
  Purchase #5 $234.42   10.104%   $258.1058   $258.11   $258.10   $258.10  
  Purchase #6 $50.26   13.507%   $57.0486   $57.05   $57.05   $57.00  
  Purchase #7 $46.26   8.531%   $50.2064   $50.21   $50.20   $50.20  
  7 purchase totals = $1,065.97       $1,160.9238   $1,160.94   $1,160.95   $1,160.80