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 $218.57   5.707%   $231.0438   $231.04   $231.05   $231.00  
  Purchase #2 $213.94   4.869%   $224.3567   $224.36   $224.35   $224.40  
  Purchase #3 $156.70   3.298%   $161.868   $161.87   $161.85   $161.90  
  Purchase #4 $69.61   8.821%   $75.7503   $75.75   $75.75   $75.80  
  Purchase #5 $52.83   11.122%   $58.7058   $58.71   $58.70   $58.70  
  Purchase #6 $104.61   4.109%   $108.9084   $108.91   $108.90   $108.90  
  Purchase #7 $125.85   7.744%   $135.5958   $135.60   $135.60   $135.60  
  7 purchase totals = $942.11       $996.2288   $996.24   $996.20   $996.30