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 $130.82   5.258%   $137.6985   $137.70   $137.70   $137.70  
  Purchase #2 $120.25   7.443%   $129.2002   $129.20   $129.20   $129.20  
  Purchase #3 $57.78   8.286%   $62.5677   $62.57   $62.55   $62.60  
  Purchase #4 $70.51   6.296%   $74.9493   $74.95   $74.95   $74.90  
  Purchase #5 $77.62   4.247%   $80.9165   $80.92   $80.90   $80.90  
  Purchase #6 $88.72   8.509%   $96.2692   $96.27   $96.25   $96.30  
  Purchase #7 $52.73   10.154%   $58.0842   $58.08   $58.10   $58.10  
  7 purchase totals = $598.43       $639.6856   $639.69   $639.65   $639.70