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 $244.83   6.104%   $259.7744   $259.77   $259.75   $259.80  
  Purchase #2 $182.56   11.587%   $203.7132   $203.71   $203.70   $203.70  
  Purchase #3 $187.36   4.457%   $195.7106   $195.71   $195.70   $195.70  
  Purchase #4 $87.96   7.874%   $94.886   $94.89   $94.90   $94.90  
  Purchase #5 $56.96   11.448%   $63.4808   $63.48   $63.50   $63.50  
  Purchase #6 $39.84   3.335%   $41.1687   $41.17   $41.15   $41.20  
  Purchase #7 $70.51   7.314%   $75.6671   $75.67   $75.65   $75.70  
  7 purchase totals = $870.02       $934.4008   $934.40   $934.35   $934.50