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 $5.50   13.201%   $6.2261   $6.23   $6.25   $6.20  
  Purchase #2 $147.94   5.771%   $156.4776   $156.48   $156.50   $156.50  
  Purchase #3 $107.80   12.825%   $121.6254   $121.63   $121.65   $121.60  
  Purchase #4 $115.95   13.602%   $131.7215   $131.72   $131.70   $131.70  
  Purchase #5 $80.76   4.535%   $84.4225   $84.42   $84.40   $84.40  
  Purchase #6 $29.73   11.411%   $33.1225   $33.12   $33.10   $33.10  
  Purchase #7 $105.95   5.871%   $112.1703   $112.17   $112.15   $112.20  
  7 purchase totals = $593.63       $645.7659   $645.77   $645.75   $645.70