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 $240.41   13.285%   $272.3485   $272.35   $272.35   $272.30  
  Purchase #2 $173.80   7.403%   $186.6664   $186.67   $186.65   $186.70  
  Purchase #3 $50.40   4.671%   $52.7542   $52.75   $52.75   $52.80  
  Purchase #4 $6.29   3.746%   $6.5256   $6.53   $6.55   $6.50  
  Purchase #5 $182.47   9.343%   $199.5182   $199.52   $199.50   $199.50  
  Purchase #6 $85.52   4.421%   $89.3008   $89.30   $89.30   $89.30  
  Purchase #7 $115.28   4.639%   $120.6278   $120.63   $120.65   $120.60  
  7 purchase totals = $854.17       $927.7415   $927.75   $927.75   $927.70