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 $227.83   13.229%   $257.9696   $257.97   $257.95   $258.00  
  Purchase #2 $161.80   7.205%   $173.4577   $173.46   $173.45   $173.50  
  Purchase #3 $244.81   4.384%   $255.5425   $255.54   $255.55   $255.50  
  Purchase #4 $216.26   7.769%   $233.0612   $233.06   $233.05   $233.10  
  Purchase #5 $126.57   6.623%   $134.9527   $134.95   $134.95   $135.00  
  Purchase #6 $122.95   9.386%   $134.4901   $134.49   $134.50   $134.50  
  Purchase #7 $240.47   7.411%   $258.2912   $258.29   $258.30   $258.30  
  7 purchase totals = $1,340.69       $1,447.7650   $1,447.76   $1,447.75   $1,447.90