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 $19.38   4.112%   $20.1769   $20.18   $20.20   $20.20  
  Purchase #2 $130.45   9.657%   $143.0476   $143.05   $143.05   $143.00  
  Purchase #3 $39.59   10.522%   $43.7557   $43.76   $43.75   $43.80  
  Purchase #4 $169.46   12.541%   $190.712   $190.71   $190.70   $190.70  
  Purchase #5 $194.96   13.212%   $220.7181   $220.72   $220.70   $220.70  
  Purchase #6 $147.56   3.826%   $153.2056   $153.21   $153.20   $153.20  
  Purchase #7 $250.53   3.163%   $258.4543   $258.45   $258.45   $258.50  
  7 purchase totals = $951.93       $1,030.0702   $1,030.08   $1,030.05   $1,030.10