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 $18.51   5.377%   $19.5053   $19.51   $19.50   $19.50  
  Purchase #2 $223.53   3.144%   $230.5578   $230.56   $230.55   $230.60  
  Purchase #3 $34.83   3.469%   $36.0383   $36.04   $36.05   $36.00  
  Purchase #4 $69.25   8.777%   $75.3281   $75.33   $75.35   $75.30  
  Purchase #5 $106.38   7.234%   $114.0755   $114.08   $114.10   $114.10  
  Purchase #6 $17.51   5.681%   $18.5047   $18.50   $18.50   $18.50  
  Purchase #7 $249.51   8.185%   $269.9324   $269.93   $269.95   $269.90  
  7 purchase totals = $719.52       $763.9421   $763.95   $764.00   $763.90