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 $12.31   4.334%   $12.8435   $12.84   $12.85   $12.80  
  Purchase #2 $19.28   7.865%   $20.7964   $20.80   $20.80   $20.80  
  Purchase #3 $175.83   4.340%   $183.461   $183.46   $183.45   $183.50  
  Purchase #4 $113.66   7.694%   $122.405   $122.41   $122.40   $122.40  
  Purchase #5 $153.27   13.245%   $173.5706   $173.57   $173.55   $173.60  
  Purchase #6 $166.36   8.821%   $181.0346   $181.03   $181.05   $181.00  
  Purchase #7 $90.26   7.478%   $97.0096   $97.01   $97.00   $97.00  
  7 purchase totals = $730.97       $791.1207   $791.12   $791.10   $791.10