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 $134.80   12.419%   $151.5408   $151.54   $151.55   $151.50  
  Purchase #2 $106.43   10.624%   $117.7371   $117.74   $117.75   $117.70  
  Purchase #3 $181.63   5.328%   $191.3072   $191.31   $191.30   $191.30  
  Purchase #4 $71.68   10.801%   $79.4222   $79.42   $79.40   $79.40  
  Purchase #5 $82.87   9.254%   $90.5388   $90.54   $90.55   $90.50  
  Purchase #6 $64.30   5.196%   $67.641   $67.64   $67.65   $67.60  
  Purchase #7 $242.31   7.710%   $260.9921   $260.99   $261.00   $261.00  
  7 purchase totals = $884.02       $959.1792   $959.18   $959.20   $959.00