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 $211.48   9.327%   $231.2047   $231.20   $231.20   $231.20  
  Purchase #2 $174.41   13.166%   $197.3728   $197.37   $197.35   $197.40  
  Purchase #3 $37.65   8.539%   $40.8649   $40.86   $40.85   $40.90  
  Purchase #4 $151.40   12.469%   $170.2781   $170.28   $170.30   $170.30  
  Purchase #5 $230.36   12.141%   $258.328   $258.33   $258.35   $258.30  
  Purchase #6 $15.37   9.271%   $16.795   $16.80   $16.80   $16.80  
  Purchase #7 $105.84   6.204%   $112.4063   $112.41   $112.40   $112.40  
  7 purchase totals = $926.51       $1,027.2498   $1,027.25   $1,027.25   $1,027.30