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 $40.85   12.674%   $46.0273   $46.03   $46.05   $46.00  
  Purchase #2 $99.70   10.367%   $110.0359   $110.04   $110.05   $110.00  
  Purchase #3 $186.66   7.472%   $200.6072   $200.61   $200.60   $200.60  
  Purchase #4 $115.59   5.831%   $122.3301   $122.33   $122.35   $122.30  
  Purchase #5 $17.40   4.824%   $18.2394   $18.24   $18.25   $18.20  
  Purchase #6 $35.27   3.784%   $36.6046   $36.60   $36.60   $36.60  
  Purchase #7 $59.37   11.669%   $66.2979   $66.30   $66.30   $66.30  
  7 purchase totals = $554.84       $600.1424   $600.15   $600.20   $600.00