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 $129.99   5.737%   $137.4475   $137.45   $137.45   $137.40  
  Purchase #2 $138.86   3.472%   $143.6812   $143.68   $143.70   $143.70  
  Purchase #3 $138.50   7.786%   $149.2836   $149.28   $149.30   $149.30  
  Purchase #4 $19.59   9.264%   $21.4048   $21.40   $21.40   $21.40  
  Purchase #5 $13.90   6.566%   $14.8127   $14.81   $14.80   $14.80  
  Purchase #6 $244.85   9.489%   $268.0838   $268.08   $268.10   $268.10  
  Purchase #7 $100.91   8.235%   $109.2199   $109.22   $109.20   $109.20  
  7 purchase totals = $786.60       $843.9335   $843.92   $843.95   $843.90