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 $7.40   11.226%   $8.2307   $8.23   $8.25   $8.20  
  Purchase #2 $13.51   11.575%   $15.0738   $15.07   $15.05   $15.10  
  Purchase #3 $77.50   7.847%   $83.5814   $83.58   $83.60   $83.60  
  Purchase #4 $75.55   7.232%   $81.0138   $81.01   $81.00   $81.00  
  Purchase #5 $161.51   8.251%   $174.8362   $174.84   $174.85   $174.80  
  Purchase #6 $124.62   7.647%   $134.1497   $134.15   $134.15   $134.10  
  Purchase #7 $19.81   10.612%   $21.9122   $21.91   $21.90   $21.90  
  7 purchase totals = $479.90       $518.7978   $518.79   $518.80   $518.70