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 $28.96   7.868%   $31.2386   $31.24   $31.25   $31.20  
  Purchase #2 $77.60   13.135%   $87.7928   $87.79   $87.80   $87.80  
  Purchase #3 $217.31   8.638%   $236.0812   $236.08   $236.10   $236.10  
  Purchase #4 $122.39   6.454%   $130.2891   $130.29   $130.30   $130.30  
  Purchase #5 $196.85   8.757%   $214.0882   $214.09   $214.10   $214.10  
  Purchase #6 $10.99   10.271%   $12.1188   $12.12   $12.10   $12.10  
  Purchase #7 $189.70   12.365%   $213.1564   $213.16   $213.15   $213.20  
  7 purchase totals = $843.80       $924.7651   $924.77   $924.80   $924.80