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 $20.31   3.616%   $21.0444   $21.04   $21.05   $21.00  
  Purchase #2 $149.31   4.476%   $155.9931   $155.99   $156.00   $156.00  
  Purchase #3 $88.59   9.580%   $97.0769   $97.08   $97.10   $97.10  
  Purchase #4 $198.27   9.182%   $216.4752   $216.48   $216.50   $216.50  
  Purchase #5 $43.64   4.852%   $45.7574   $45.76   $45.75   $45.80  
  Purchase #6 $151.31   10.324%   $166.9312   $166.93   $166.95   $166.90  
  Purchase #7 $160.66   12.194%   $180.2509   $180.25   $180.25   $180.30  
  7 purchase totals = $812.09       $883.5291   $883.53   $883.60   $883.60