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 $115.70   13.395%   $131.198   $131.20   $131.20   $131.20  
  Purchase #2 $71.48   13.186%   $80.9054   $80.91   $80.90   $80.90  
  Purchase #3 $86.67   11.783%   $96.8823   $96.88   $96.90   $96.90  
  Purchase #4 $126.44   5.513%   $133.4106   $133.41   $133.40   $133.40  
  Purchase #5 $241.50   4.724%   $252.9085   $252.91   $252.90   $252.90  
  Purchase #6 $206.35   5.413%   $217.5197   $217.52   $217.50   $217.50  
  Purchase #7 $82.59   4.182%   $86.0439   $86.04   $86.05   $86.00  
  7 purchase totals = $930.73       $998.8684   $998.87   $998.85   $998.80