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 $75.77   13.162%   $85.7428   $85.74   $85.75   $85.70  
  Purchase #2 $226.35   13.792%   $257.5682   $257.57   $257.55   $257.60  
  Purchase #3 $175.42   4.514%   $183.3385   $183.34   $183.35   $183.30  
  Purchase #4 $43.44   12.253%   $48.7627   $48.76   $48.75   $48.80  
  Purchase #5 $180.60   7.119%   $193.4569   $193.46   $193.45   $193.50  
  Purchase #6 $37.26   6.504%   $39.6834   $39.68   $39.70   $39.70  
  Purchase #7 $44.79   8.596%   $48.6401   $48.64   $48.65   $48.60  
  7 purchase totals = $783.63       $857.1926   $857.19   $857.20   $857.20