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 $233.48   6.776%   $249.3006   $249.30   $249.30   $249.30  
  Purchase #2 $91.58   7.105%   $98.0868   $98.09   $98.10   $98.10  
  Purchase #3 $38.97   11.113%   $43.3007   $43.30   $43.30   $43.30  
  Purchase #4 $83.44   11.679%   $93.185   $93.19   $93.20   $93.20  
  Purchase #5 $234.79   12.148%   $263.3123   $263.31   $263.30   $263.30  
  Purchase #6 $60.92   10.312%   $67.2021   $67.20   $67.20   $67.20  
  Purchase #7 $161.90   11.456%   $180.4473   $180.45   $180.45   $180.40  
  7 purchase totals = $905.08       $994.8348   $994.84   $994.85   $994.80