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 $218.34   13.722%   $248.3006   $248.30   $248.30   $248.30  
  Purchase #2 $216.37   6.313%   $230.0294   $230.03   $230.05   $230.00  
  Purchase #3 $64.32   5.156%   $67.6363   $67.64   $67.65   $67.60  
  Purchase #4 $31.93   12.148%   $35.8089   $35.81   $35.80   $35.80  
  Purchase #5 $70.32   7.857%   $75.845   $75.85   $75.85   $75.80  
  Purchase #6 $60.45   9.533%   $66.2127   $66.21   $66.20   $66.20  
  Purchase #7 $96.60   7.275%   $103.6277   $103.63   $103.65   $103.60  
  7 purchase totals = $758.33       $827.4606   $827.47   $827.50   $827.30