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 $27.45   4.438%   $28.6682   $28.67   $28.65   $28.70  
  Purchase #2 $18.59   12.135%   $20.8459   $20.85   $20.85   $20.80  
  Purchase #3 $20.84   10.525%   $23.0334   $23.03   $23.05   $23.00  
  Purchase #4 $99.69   3.275%   $102.9548   $102.95   $102.95   $103.00  
  Purchase #5 $249.56   8.833%   $271.6036   $271.60   $271.60   $271.60  
  Purchase #6 $185.48   13.596%   $210.6979   $210.70   $210.70   $210.70  
  Purchase #7 $182.48   4.788%   $191.2171   $191.22   $191.20   $191.20  
  7 purchase totals = $784.09       $849.0209   $849.02   $849.00   $849.00