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 $51.73   13.288%   $58.6039   $58.60   $58.60   $58.60  
  Purchase #2 $75.37   5.528%   $79.5365   $79.54   $79.55   $79.50  
  Purchase #3 $227.30   11.715%   $253.9282   $253.93   $253.95   $253.90  
  Purchase #4 $200.77   9.219%   $219.279   $219.28   $219.30   $219.30  
  Purchase #5 $36.64   9.340%   $40.0622   $40.06   $40.05   $40.10  
  Purchase #6 $43.36   3.829%   $45.0203   $45.02   $45.00   $45.00  
  Purchase #7 $78.64   3.131%   $81.1022   $81.10   $81.10   $81.10  
  7 purchase totals = $713.81       $777.5323   $777.53   $777.55   $777.50