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 $122.69   4.614%   $128.3509   $128.35   $128.35   $128.40  
  Purchase #2 $145.40   6.340%   $154.6184   $154.62   $154.60   $154.60  
  Purchase #3 $102.27   13.636%   $116.2155   $116.22   $116.20   $116.20  
  Purchase #4 $37.81   5.749%   $39.9837   $39.98   $40.00   $40.00  
  Purchase #5 $79.97   9.423%   $87.5056   $87.51   $87.50   $87.50  
  Purchase #6 $30.80   6.727%   $32.8719   $32.87   $32.85   $32.90  
  Purchase #7 $117.78   11.469%   $131.2882   $131.29   $131.30   $131.30  
  7 purchase totals = $636.72       $690.8342   $690.84   $690.80   $690.90