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 $238.61   13.137%   $269.9562   $269.96   $269.95   $270.00  
  Purchase #2 $51.85   12.474%   $58.3178   $58.32   $58.30   $58.30  
  Purchase #3 $43.31   12.134%   $48.5652   $48.57   $48.55   $48.60  
  Purchase #4 $55.76   6.538%   $59.4056   $59.41   $59.40   $59.40  
  Purchase #5 $204.91   6.566%   $218.3644   $218.36   $218.35   $218.40  
  Purchase #6 $192.88   10.139%   $212.4361   $212.44   $212.45   $212.40  
  Purchase #7 $42.67   13.593%   $48.4701   $48.47   $48.45   $48.50  
  7 purchase totals = $829.99       $915.5154   $915.53   $915.45   $915.60