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 $226.44   12.168%   $253.9932   $253.99   $254.00   $254.00  
  Purchase #2 $235.61   13.139%   $266.5668   $266.57   $266.55   $266.60  
  Purchase #3 $38.71   4.200%   $40.3358   $40.34   $40.35   $40.30  
  Purchase #4 $81.62   8.257%   $88.3594   $88.36   $88.35   $88.40  
  Purchase #5 $181.47   11.456%   $202.2592   $202.26   $202.25   $202.30  
  Purchase #6 $135.64   4.468%   $141.7004   $141.70   $141.70   $141.70  
  Purchase #7 $109.51   11.608%   $122.2219   $122.22   $122.20   $122.20  
  7 purchase totals = $1,009.00       $1,115.4367   $1,115.44   $1,115.40   $1,115.50