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 $224.93   12.452%   $252.9383   $252.94   $252.95   $252.90  
  Purchase #2 $128.84   10.573%   $142.4623   $142.46   $142.45   $142.50  
  Purchase #3 $81.28   6.716%   $86.7388   $86.74   $86.75   $86.70  
  Purchase #4 $167.84   5.629%   $177.2877   $177.29   $177.30   $177.30  
  Purchase #5 $49.48   11.677%   $55.2578   $55.26   $55.25   $55.30  
  Purchase #6 $147.84   10.430%   $163.2597   $163.26   $163.25   $163.30  
  Purchase #7 $13.98   13.328%   $15.8433   $15.84   $15.85   $15.80  
  7 purchase totals = $814.19       $893.7879   $893.79   $893.80   $893.80