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 $15.29   3.130%   $15.7686   $15.77   $15.75   $15.80  
  Purchase #2 $28.77   12.117%   $32.2561   $32.26   $32.25   $32.30  
  Purchase #3 $213.60   7.699%   $230.0451   $230.05   $230.05   $230.00  
  Purchase #4 $101.41   5.556%   $107.0443   $107.04   $107.05   $107.00  
  Purchase #5 $73.70   8.584%   $80.0264   $80.03   $80.05   $80.00  
  Purchase #6 $69.61   13.746%   $79.1786   $79.18   $79.20   $79.20  
  Purchase #7 $137.55   12.275%   $154.4343   $154.43   $154.45   $154.40  
  7 purchase totals = $639.93       $698.7534   $698.76   $698.80   $698.70