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 $20.74   3.112%   $21.3854   $21.39   $21.40   $21.40  
  Purchase #2 $129.79   10.242%   $143.0831   $143.08   $143.10   $143.10  
  Purchase #3 $51.35   4.523%   $53.6726   $53.67   $53.65   $53.70  
  Purchase #4 $115.91   7.834%   $124.9904   $124.99   $125.00   $125.00  
  Purchase #5 $136.69   11.249%   $152.0663   $152.07   $152.05   $152.10  
  Purchase #6 $15.48   6.661%   $16.5111   $16.51   $16.50   $16.50  
  Purchase #7 $121.26   13.803%   $137.9975   $138.00   $138.00   $138.00  
  7 purchase totals = $591.22       $649.7064   $649.71   $649.70   $649.80