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 $159.66   13.756%   $181.6228   $181.62   $181.60   $181.60  
  Purchase #2 $133.32   8.660%   $144.8655   $144.87   $144.85   $144.90  
  Purchase #3 $115.93   4.439%   $121.0761   $121.08   $121.10   $121.10  
  Purchase #4 $162.27   6.732%   $173.194   $173.19   $173.20   $173.20  
  Purchase #5 $91.58   3.542%   $94.8238   $94.82   $94.80   $94.80  
  Purchase #6 $120.90   10.695%   $133.8303   $133.83   $133.85   $133.80  
  Purchase #7 $162.51   9.210%   $177.4772   $177.48   $177.50   $177.50  
  7 purchase totals = $946.17       $1,026.8897   $1,026.89   $1,026.90   $1,026.90