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 $249.29   3.549%   $258.1373   $258.14   $258.15   $258.10  
  Purchase #2 $56.73   13.429%   $64.3483   $64.35   $64.35   $64.30  
  Purchase #3 $214.46   5.134%   $225.4704   $225.47   $225.45   $225.50  
  Purchase #4 $222.86   13.293%   $252.4848   $252.48   $252.50   $252.50  
  Purchase #5 $75.93   11.259%   $84.479   $84.48   $84.50   $84.50  
  Purchase #6 $55.84   12.334%   $62.7273   $62.73   $62.75   $62.70  
  Purchase #7 $108.45   9.731%   $119.0033   $119.00   $119.00   $119.00  
  7 purchase totals = $983.56       $1,066.6504   $1,066.65   $1,066.70   $1,066.60