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 $28.40   5.383%   $29.9288   $29.93   $29.95   $29.90  
  Purchase #2 $55.49   10.622%   $61.3841   $61.38   $61.40   $61.40  
  Purchase #3 $129.57   13.812%   $147.4662   $147.47   $147.45   $147.50  
  Purchase #4 $246.70   3.485%   $255.2975   $255.30   $255.30   $255.30  
  Purchase #5 $99.73   5.363%   $105.0785   $105.08   $105.10   $105.10  
  Purchase #6 $228.97   9.312%   $250.2917   $250.29   $250.30   $250.30  
  Purchase #7 $62.66   12.842%   $70.7068   $70.71   $70.70   $70.70  
  7 purchase totals = $851.52       $920.1536   $920.16   $920.20   $920.20