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 $248.31   10.332%   $273.9654   $273.97   $273.95   $274.00  
  Purchase #2 $172.63   9.314%   $188.7088   $188.71   $188.70   $188.70  
  Purchase #3 $135.28   5.176%   $142.2821   $142.28   $142.30   $142.30  
  Purchase #4 $162.36   10.811%   $179.9127   $179.91   $179.90   $179.90  
  Purchase #5 $122.46   9.693%   $134.33   $134.33   $134.35   $134.30  
  Purchase #6 $204.58   9.782%   $224.592   $224.59   $224.60   $224.60  
  Purchase #7 $220.36   5.816%   $233.1761   $233.18   $233.20   $233.20  
  7 purchase totals = $1,265.98       $1,376.9671   $1,376.97   $1,377.00   $1,377.00