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 $12.51   10.704%   $13.8491   $13.85   $13.85   $13.80  
  Purchase #2 $194.48   7.578%   $209.2177   $209.22   $209.20   $209.20  
  Purchase #3 $114.50   6.230%   $121.6334   $121.63   $121.65   $121.60  
  Purchase #4 $100.50   9.525%   $110.0726   $110.07   $110.05   $110.10  
  Purchase #5 $76.80   3.582%   $79.551   $79.55   $79.55   $79.60  
  Purchase #6 $14.32   12.765%   $16.1479   $16.15   $16.15   $16.10  
  Purchase #7 $86.72   6.113%   $92.0212   $92.02   $92.00   $92.00  
  7 purchase totals = $599.83       $642.4929   $642.49   $642.45   $642.40