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 $157.93   13.227%   $178.8194   $178.82   $178.80   $178.80  
  Purchase #2 $73.68   12.605%   $82.9674   $82.97   $82.95   $83.00  
  Purchase #3 $79.33   8.364%   $85.9652   $85.97   $85.95   $86.00  
  Purchase #4 $250.57   7.592%   $269.5933   $269.59   $269.60   $269.60  
  Purchase #5 $213.98   13.426%   $242.709   $242.71   $242.70   $242.70  
  Purchase #6 $163.25   3.580%   $169.0944   $169.09   $169.10   $169.10  
  Purchase #7 $220.75   13.463%   $250.4696   $250.47   $250.45   $250.50  
  7 purchase totals = $1,159.49       $1,279.6183   $1,279.62   $1,279.55   $1,279.70