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 $27.79   10.375%   $30.6732   $30.67   $30.65   $30.70  
  Purchase #2 $100.87   7.439%   $108.3737   $108.37   $108.35   $108.40  
  Purchase #3 $115.92   3.623%   $120.1198   $120.12   $120.10   $120.10  
  Purchase #4 $16.97   10.724%   $18.7899   $18.79   $18.80   $18.80  
  Purchase #5 $168.58   4.740%   $176.5707   $176.57   $176.55   $176.60  
  Purchase #6 $36.77   6.794%   $39.2682   $39.27   $39.25   $39.30  
  Purchase #7 $40.56   8.578%   $44.0392   $44.04   $44.05   $44.00  
  7 purchase totals = $507.46       $537.8347   $537.83   $537.75   $537.90