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 $126.53   8.198%   $136.9029   $136.90   $136.90   $136.90  
  Purchase #2 $227.67   8.505%   $247.0333   $247.03   $247.05   $247.00  
  Purchase #3 $72.33   9.505%   $79.205   $79.21   $79.20   $79.20  
  Purchase #4 $104.82   4.787%   $109.8377   $109.84   $109.85   $109.80  
  Purchase #5 $232.93   7.592%   $250.614   $250.61   $250.60   $250.60  
  Purchase #6 $244.80   7.206%   $262.4403   $262.44   $262.45   $262.40  
  Purchase #7 $177.69   5.773%   $187.948   $187.95   $187.95   $187.90  
  7 purchase totals = $1,186.77       $1,273.9812   $1,273.98   $1,274.00   $1,273.80