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 $12.41   3.231%   $12.811   $12.81   $12.80   $12.80  
  Purchase #2 $137.58   12.647%   $154.9797   $154.98   $155.00   $155.00  
  Purchase #3 $250.96   13.498%   $284.8346   $284.83   $284.85   $284.80  
  Purchase #4 $40.56   6.373%   $43.1449   $43.14   $43.15   $43.10  
  Purchase #5 $78.51   7.181%   $84.1478   $84.15   $84.15   $84.10  
  Purchase #6 $32.68   9.352%   $35.7362   $35.74   $35.75   $35.70  
  Purchase #7 $20.77   10.596%   $22.9708   $22.97   $22.95   $23.00  
  7 purchase totals = $573.47       $638.6250   $638.62   $638.65   $638.50