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 $169.37   13.146%   $191.6354   $191.64   $191.65   $191.60  
  Purchase #2 $83.73   12.114%   $93.8731   $93.87   $93.85   $93.90  
  Purchase #3 $134.35   9.460%   $147.0595   $147.06   $147.05   $147.10  
  Purchase #4 $244.34   6.811%   $260.982   $260.98   $261.00   $261.00  
  Purchase #5 $44.62   12.789%   $50.3265   $50.33   $50.35   $50.30  
  Purchase #6 $29.84   8.833%   $32.4758   $32.48   $32.50   $32.50  
  Purchase #7 $71.96   12.739%   $81.127   $81.13   $81.15   $81.10  
  7 purchase totals = $778.21       $857.4793   $857.49   $857.55   $857.50