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 $79.95   8.339%   $86.617   $86.62   $86.60   $86.60  
  Purchase #2 $232.61   11.314%   $258.9275   $258.93   $258.95   $258.90  
  Purchase #3 $16.60   12.168%   $18.6199   $18.62   $18.60   $18.60  
  Purchase #4 $87.39   6.454%   $93.0302   $93.03   $93.05   $93.00  
  Purchase #5 $34.51   3.254%   $35.633   $35.63   $35.65   $35.60  
  Purchase #6 $138.82   10.693%   $153.664   $153.66   $153.65   $153.70  
  Purchase #7 $143.96   6.686%   $153.5852   $153.59   $153.60   $153.60  
  7 purchase totals = $733.84       $800.0768   $800.08   $800.10   $800.00