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 $243.68   12.250%   $273.5308   $273.53   $273.55   $273.50  
  Purchase #2 $218.93   12.188%   $245.6132   $245.61   $245.60   $245.60  
  Purchase #3 $194.39   9.557%   $212.9679   $212.97   $212.95   $213.00  
  Purchase #4 $248.74   8.496%   $269.873   $269.87   $269.85   $269.90  
  Purchase #5 $60.66   8.871%   $66.0411   $66.04   $66.05   $66.00  
  Purchase #6 $47.81   10.740%   $52.9448   $52.94   $52.95   $52.90  
  Purchase #7 $220.45   5.560%   $232.707   $232.71   $232.70   $232.70  
  7 purchase totals = $1,234.66       $1,353.6778   $1,353.67   $1,353.65   $1,353.60