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 $21.87   3.708%   $22.6809   $22.68   $22.70   $22.70  
  Purchase #2 $29.60   4.202%   $30.8438   $30.84   $30.85   $30.80  
  Purchase #3 $201.50   12.712%   $227.1147   $227.11   $227.10   $227.10  
  Purchase #4 $202.28   12.718%   $228.006   $228.01   $228.00   $228.00  
  Purchase #5 $193.88   10.777%   $214.7744   $214.77   $214.75   $214.80  
  Purchase #6 $12.27   6.350%   $13.0491   $13.05   $13.05   $13.00  
  Purchase #7 $79.46   8.635%   $86.3214   $86.32   $86.30   $86.30  
  7 purchase totals = $740.86       $822.7903   $822.78   $822.75   $822.70