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 $25.84   11.766%   $28.8803   $28.88   $28.90   $28.90  
  Purchase #2 $117.30   9.253%   $128.1538   $128.15   $128.15   $128.20  
  Purchase #3 $73.97   10.609%   $81.8175   $81.82   $81.80   $81.80  
  Purchase #4 $51.54   8.195%   $55.7637   $55.76   $55.75   $55.80  
  Purchase #5 $50.79   6.554%   $54.1188   $54.12   $54.10   $54.10  
  Purchase #6 $104.78   11.870%   $117.2174   $117.22   $117.20   $117.20  
  Purchase #7 $205.46   7.115%   $220.0785   $220.08   $220.10   $220.10  
  7 purchase totals = $629.68       $686.0300   $686.03   $686.00   $686.10