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 $25.27   11.275%   $28.1192   $28.12   $28.10   $28.10  
  Purchase #2 $16.66   7.869%   $17.971   $17.97   $17.95   $18.00  
  Purchase #3 $92.60   10.350%   $102.1841   $102.18   $102.20   $102.20  
  Purchase #4 $233.93   6.387%   $248.8711   $248.87   $248.85   $248.90  
  Purchase #5 $12.32   5.519%   $12.9999   $13.00   $13.00   $13.00  
  Purchase #6 $116.93   11.857%   $130.7944   $130.79   $130.80   $130.80  
  Purchase #7 $191.76   8.378%   $207.8257   $207.83   $207.85   $207.80  
  7 purchase totals = $689.47       $748.7654   $748.76   $748.75   $748.80