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 $25.59   13.350%   $29.0063   $29.01   $29.00   $29.00  
  Purchase #2 $135.49   7.633%   $145.832   $145.83   $145.85   $145.80  
  Purchase #3 $188.70   13.735%   $214.6179   $214.62   $214.60   $214.60  
  Purchase #4 $77.49   11.158%   $86.1363   $86.14   $86.15   $86.10  
  Purchase #5 $188.79   9.682%   $207.0686   $207.07   $207.05   $207.10  
  Purchase #6 $29.53   8.871%   $32.1496   $32.15   $32.15   $32.10  
  Purchase #7 $110.53   12.644%   $124.5054   $124.51   $124.50   $124.50  
  7 purchase totals = $756.12       $839.3161   $839.33   $839.30   $839.20