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 $100.78   7.230%   $108.0664   $108.07   $108.05   $108.10  
  Purchase #2 $10.35   10.199%   $11.4056   $11.41   $11.40   $11.40  
  Purchase #3 $133.80   3.720%   $138.7774   $138.78   $138.80   $138.80  
  Purchase #4 $38.50   9.826%   $42.283   $42.28   $42.30   $42.30  
  Purchase #5 $174.84   5.191%   $183.9159   $183.92   $183.90   $183.90  
  Purchase #6 $117.39   9.683%   $128.7569   $128.76   $128.75   $128.80  
  Purchase #7 $31.92   11.829%   $35.6958   $35.70   $35.70   $35.70  
  7 purchase totals = $607.58       $648.9010   $648.92   $648.90   $649.00