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 $148.48   11.803%   $166.0051   $166.01   $166.00   $166.00  
  Purchase #2 $63.93   6.352%   $67.9908   $67.99   $68.00   $68.00  
  Purchase #3 $186.98   11.579%   $208.6304   $208.63   $208.65   $208.60  
  Purchase #4 $249.69   13.455%   $283.2858   $283.29   $283.30   $283.30  
  Purchase #5 $103.75   10.691%   $114.8419   $114.84   $114.85   $114.80  
  Purchase #6 $54.78   4.257%   $57.112   $57.11   $57.10   $57.10  
  Purchase #7 $173.70   10.360%   $191.6953   $191.70   $191.70   $191.70  
  7 purchase totals = $981.31       $1,089.5613   $1,089.57   $1,089.60   $1,089.50