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 $121.77   10.439%   $134.4816   $134.48   $134.50   $134.50  
  Purchase #2 $14.37   7.295%   $15.4183   $15.42   $15.40   $15.40  
  Purchase #3 $159.39   13.477%   $180.871   $180.87   $180.85   $180.90  
  Purchase #4 $95.92   12.186%   $107.6088   $107.61   $107.60   $107.60  
  Purchase #5 $99.25   9.185%   $108.3661   $108.37   $108.35   $108.40  
  Purchase #6 $249.44   10.587%   $275.8482   $275.85   $275.85   $275.80  
  Purchase #7 $162.29   8.508%   $176.0976   $176.10   $176.10   $176.10  
  7 purchase totals = $902.43       $998.6916   $998.70   $998.65   $998.70