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 $227.95   12.206%   $255.7736   $255.77   $255.75   $255.80  
  Purchase #2 $12.87   13.376%   $14.5915   $14.59   $14.60   $14.60  
  Purchase #3 $173.25   8.819%   $188.5289   $188.53   $188.55   $188.50  
  Purchase #4 $173.28   3.295%   $178.9896   $178.99   $179.00   $179.00  
  Purchase #5 $170.95   10.714%   $189.2656   $189.27   $189.25   $189.30  
  Purchase #6 $190.81   10.201%   $210.2745   $210.27   $210.25   $210.30  
  Purchase #7 $141.92   12.652%   $159.8757   $159.88   $159.90   $159.90  
  7 purchase totals = $1,091.03       $1,197.2994   $1,197.30   $1,197.30   $1,197.40