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 $18.78   6.700%   $20.0383   $20.04   $20.05   $20.00  
  Purchase #2 $197.83   3.224%   $204.208   $204.21   $204.20   $204.20  
  Purchase #3 $79.26   12.863%   $89.4552   $89.46   $89.45   $89.50  
  Purchase #4 $24.64   5.647%   $26.0314   $26.03   $26.05   $26.00  
  Purchase #5 $173.88   13.302%   $197.0095   $197.01   $197.00   $197.00  
  Purchase #6 $69.83   11.121%   $77.5958   $77.60   $77.60   $77.60  
  Purchase #7 $28.37   13.766%   $32.2754   $32.28   $32.30   $32.30  
  7 purchase totals = $592.59       $646.6136   $646.63   $646.65   $646.60