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 $19.73   8.691%   $21.4447   $21.44   $21.45   $21.40  
  Purchase #2 $89.31   12.282%   $100.2791   $100.28   $100.30   $100.30  
  Purchase #3 $77.53   3.224%   $80.0296   $80.03   $80.05   $80.00  
  Purchase #4 $107.32   3.577%   $111.1588   $111.16   $111.15   $111.20  
  Purchase #5 $8.36   6.181%   $8.8767   $8.88   $8.90   $8.90  
  Purchase #6 $107.65   13.220%   $121.8813   $121.88   $121.90   $121.90  
  Purchase #7 $221.62   7.141%   $237.4459   $237.45   $237.45   $237.40  
  7 purchase totals = $631.52       $681.1161   $681.12   $681.20   $681.10