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 $107.72   11.126%   $119.7049   $119.70   $119.70   $119.70  
  Purchase #2 $89.87   4.779%   $94.1649   $94.16   $94.15   $94.20  
  Purchase #3 $151.75   7.379%   $162.9476   $162.95   $162.95   $162.90  
  Purchase #4 $179.94   9.467%   $196.9749   $196.97   $196.95   $197.00  
  Purchase #5 $195.74   3.484%   $202.5596   $202.56   $202.55   $202.60  
  Purchase #6 $121.68   11.116%   $135.2059   $135.21   $135.20   $135.20  
  Purchase #7 $122.48   4.516%   $128.0112   $128.01   $128.00   $128.00  
  7 purchase totals = $969.18       $1,039.5690   $1,039.56   $1,039.50   $1,039.60