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 $172.84   4.187%   $180.0768   $180.08   $180.10   $180.10  
  Purchase #2 $27.63   9.247%   $30.1849   $30.18   $30.20   $30.20  
  Purchase #3 $79.31   10.368%   $87.5329   $87.53   $87.55   $87.50  
  Purchase #4 $245.76   8.497%   $266.6422   $266.64   $266.65   $266.60  
  Purchase #5 $18.25   8.493%   $19.8   $19.80   $19.80   $19.80  
  Purchase #6 $26.86   10.638%   $29.7174   $29.72   $29.70   $29.70  
  Purchase #7 $202.94   13.237%   $229.8032   $229.80   $229.80   $229.80  
  7 purchase totals = $773.59       $843.7574   $843.75   $843.80   $843.70