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 $7.99   13.306%   $9.0531   $9.05   $9.05   $9.10  
  Purchase #2 $75.55   3.744%   $78.3786   $78.38   $78.40   $78.40  
  Purchase #3 $17.45   13.691%   $19.8391   $19.84   $19.85   $19.80  
  Purchase #4 $41.36   8.118%   $44.7176   $44.72   $44.70   $44.70  
  Purchase #5 $55.25   7.173%   $59.2131   $59.21   $59.20   $59.20  
  Purchase #6 $216.41   6.763%   $231.0458   $231.05   $231.05   $231.00  
  Purchase #7 $66.86   5.823%   $70.7533   $70.75   $70.75   $70.80  
  7 purchase totals = $480.87       $513.0006   $513.00   $513.00   $513.00