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 $48.38   4.762%   $50.6839   $50.68   $50.70   $50.70  
  Purchase #2 $172.96   12.129%   $193.9383   $193.94   $193.95   $193.90  
  Purchase #3 $79.55   8.412%   $86.2417   $86.24   $86.25   $86.20  
  Purchase #4 $188.53   7.653%   $202.9582   $202.96   $202.95   $203.00  
  Purchase #5 $26.93   4.749%   $28.2089   $28.21   $28.20   $28.20  
  Purchase #6 $49.88   5.659%   $52.7027   $52.70   $52.70   $52.70  
  Purchase #7 $45.33   13.112%   $51.2737   $51.27   $51.25   $51.30  
  7 purchase totals = $611.56       $666.0074   $666.00   $666.00   $666.00