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 $28.80   7.872%   $31.0671   $31.07   $31.05   $31.10  
  Purchase #2 $164.63   4.227%   $171.5889   $171.59   $171.60   $171.60  
  Purchase #3 $153.41   11.454%   $170.9816   $170.98   $171.00   $171.00  
  Purchase #4 $79.40   6.152%   $84.2847   $84.28   $84.30   $84.30  
  Purchase #5 $89.62   7.614%   $96.4437   $96.44   $96.45   $96.40  
  Purchase #6 $6.36   3.728%   $6.5971   $6.60   $6.60   $6.60  
  Purchase #7 $33.50   13.333%   $37.9666   $37.97   $37.95   $38.00  
  7 purchase totals = $555.72       $598.9297   $598.93   $598.95   $599.00