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 $44.37   13.361%   $50.2983   $50.30   $50.30   $50.30  
  Purchase #2 $109.94   7.300%   $117.9656   $117.97   $117.95   $118.00  
  Purchase #3 $235.39   11.665%   $262.8482   $262.85   $262.85   $262.80  
  Purchase #4 $250.63   8.150%   $271.0563   $271.06   $271.05   $271.10  
  Purchase #5 $66.89   6.262%   $71.0787   $71.08   $71.10   $71.10  
  Purchase #6 $30.76   9.380%   $33.6453   $33.65   $33.65   $33.60  
  Purchase #7 $47.70   6.372%   $50.7394   $50.74   $50.75   $50.70  
  7 purchase totals = $785.68       $857.6318   $857.65   $857.65   $857.60