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 $249.95   11.741%   $279.2966   $279.30   $279.30   $279.30  
  Purchase #2 $211.77   4.193%   $220.6495   $220.65   $220.65   $220.60  
  Purchase #3 $136.29   5.353%   $143.5856   $143.59   $143.60   $143.60  
  Purchase #4 $145.60   5.270%   $153.2731   $153.27   $153.25   $153.30  
  Purchase #5 $87.72   10.856%   $97.2429   $97.24   $97.25   $97.20  
  Purchase #6 $62.31   12.817%   $70.2963   $70.30   $70.30   $70.30  
  Purchase #7 $35.44   5.131%   $37.2584   $37.26   $37.25   $37.30  
  7 purchase totals = $929.08       $1,001.6024   $1,001.61   $1,001.60   $1,001.60