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 $20.79   6.781%   $22.1998   $22.20   $22.20   $22.20  
  Purchase #2 $134.33   6.592%   $143.185   $143.19   $143.20   $143.20  
  Purchase #3 $44.83   13.411%   $50.8422   $50.84   $50.85   $50.80  
  Purchase #4 $73.43   10.491%   $81.1335   $81.13   $81.15   $81.10  
  Purchase #5 $20.36   12.475%   $22.8999   $22.90   $22.90   $22.90  
  Purchase #6 $141.61   4.390%   $147.8267   $147.83   $147.85   $147.80  
  Purchase #7 $14.55   9.176%   $15.8851   $15.89   $15.90   $15.90  
  7 purchase totals = $449.90       $483.9722   $483.98   $484.05   $483.90