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 $21.94   6.719%   $23.4141   $23.41   $23.40   $23.40  
  Purchase #2 $28.60   9.710%   $31.3771   $31.38   $31.40   $31.40  
  Purchase #3 $122.55   10.269%   $135.1347   $135.13   $135.15   $135.10  
  Purchase #4 $50.71   11.472%   $56.5275   $56.53   $56.55   $56.50  
  Purchase #5 $112.72   7.212%   $120.8494   $120.85   $120.85   $120.80  
  Purchase #6 $238.32   5.379%   $251.1392   $251.14   $251.15   $251.10  
  Purchase #7 $17.64   9.411%   $19.3001   $19.30   $19.30   $19.30  
  7 purchase totals = $592.48       $637.7421   $637.74   $637.80   $637.60