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 $25.95   10.396%   $28.6478   $28.65   $28.65   $28.60  
  Purchase #2 $75.57   9.144%   $82.4801   $82.48   $82.50   $82.50  
  Purchase #3 $221.41   7.829%   $238.7442   $238.74   $238.75   $238.70  
  Purchase #4 $220.44   10.417%   $243.4032   $243.40   $243.40   $243.40  
  Purchase #5 $12.99   12.812%   $14.6543   $14.65   $14.65   $14.70  
  Purchase #6 $52.38   7.246%   $56.1755   $56.18   $56.20   $56.20  
  Purchase #7 $137.83   6.677%   $147.0329   $147.03   $147.05   $147.00  
  7 purchase totals = $746.57       $811.1380   $811.13   $811.20   $811.10