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 $34.49   11.377%   $38.4139   $38.41   $38.40   $38.40  
  Purchase #2 $193.82   6.728%   $206.8602   $206.86   $206.85   $206.90  
  Purchase #3 $114.66   7.659%   $123.4418   $123.44   $123.45   $123.40  
  Purchase #4 $185.71   6.111%   $197.0587   $197.06   $197.05   $197.10  
  Purchase #5 $187.79   8.376%   $203.5193   $203.52   $203.50   $203.50  
  Purchase #6 $113.49   6.661%   $121.0496   $121.05   $121.05   $121.00  
  Purchase #7 $114.37   9.608%   $125.3587   $125.36   $125.35   $125.40  
  7 purchase totals = $944.33       $1,015.7022   $1,015.70   $1,015.65   $1,015.70