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 $21.82   13.203%   $24.7009   $24.70   $24.70   $24.70  
  Purchase #2 $196.41   4.713%   $205.6668   $205.67   $205.65   $205.70  
  Purchase #3 $240.68   7.672%   $259.145   $259.15   $259.15   $259.10  
  Purchase #4 $11.86   13.454%   $13.4556   $13.46   $13.45   $13.50  
  Purchase #5 $232.26   13.326%   $263.211   $263.21   $263.20   $263.20  
  Purchase #6 $108.31   8.589%   $117.6127   $117.61   $117.60   $117.60  
  Purchase #7 $44.39   4.816%   $46.5278   $46.53   $46.55   $46.50  
  7 purchase totals = $855.73       $930.3198   $930.33   $930.30   $930.30