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 $244.95   12.102%   $274.5938   $274.59   $274.60   $274.60  
  Purchase #2 $232.30   9.797%   $255.0584   $255.06   $255.05   $255.10  
  Purchase #3 $115.99   12.237%   $130.1837   $130.18   $130.20   $130.20  
  Purchase #4 $98.47   12.223%   $110.506   $110.51   $110.50   $110.50  
  Purchase #5 $11.48   12.751%   $12.9438   $12.94   $12.95   $12.90  
  Purchase #6 $103.93   7.144%   $111.3548   $111.35   $111.35   $111.40  
  Purchase #7 $95.80   5.543%   $101.1102   $101.11   $101.10   $101.10  
  7 purchase totals = $902.92       $995.7507   $995.74   $995.75   $995.80