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 $243.25   5.141%   $255.7555   $255.76   $255.75   $255.80  
  Purchase #2 $62.88   3.792%   $65.2644   $65.26   $65.25   $65.30  
  Purchase #3 $104.29   4.843%   $109.3408   $109.34   $109.35   $109.30  
  Purchase #4 $155.31   10.442%   $171.5275   $171.53   $171.55   $171.50  
  Purchase #5 $162.75   12.235%   $182.6625   $182.66   $182.65   $182.70  
  Purchase #6 $151.71   6.358%   $161.3557   $161.36   $161.35   $161.40  
  Purchase #7 $38.34   12.328%   $43.0666   $43.07   $43.05   $43.10  
  7 purchase totals = $918.53       $988.9730   $988.98   $988.95   $989.10