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 $235.99   13.716%   $268.3584   $268.36   $268.35   $268.40  
  Purchase #2 $182.60   8.772%   $198.6177   $198.62   $198.60   $198.60  
  Purchase #3 $27.34   5.524%   $28.8503   $28.85   $28.85   $28.90  
  Purchase #4 $204.51   10.744%   $226.4826   $226.48   $226.50   $226.50  
  Purchase #5 $134.75   11.414%   $150.1304   $150.13   $150.15   $150.10  
  Purchase #6 $202.42   4.867%   $212.2718   $212.27   $212.25   $212.30  
  Purchase #7 $161.30   5.660%   $170.4296   $170.43   $170.45   $170.40  
  7 purchase totals = $1,148.91       $1,255.1408   $1,255.14   $1,255.15   $1,255.20