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 $23.43   13.676%   $26.6343   $26.63   $26.65   $26.60  
  Purchase #2 $48.66   9.652%   $53.3567   $53.36   $53.35   $53.40  
  Purchase #3 $236.33   5.585%   $249.529   $249.53   $249.55   $249.50  
  Purchase #4 $165.45   5.703%   $174.8856   $174.89   $174.90   $174.90  
  Purchase #5 $91.39   12.691%   $102.9883   $102.99   $103.00   $103.00  
  Purchase #6 $152.83   9.469%   $167.3015   $167.30   $167.30   $167.30  
  Purchase #7 $5.84   13.208%   $6.6113   $6.61   $6.60   $6.60  
  7 purchase totals = $723.93       $781.3067   $781.31   $781.35   $781.30