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 $15.72   4.245%   $16.3873   $16.39   $16.40   $16.40  
  Purchase #2 $46.34   12.711%   $52.2303   $52.23   $52.25   $52.20  
  Purchase #3 $184.30   5.714%   $194.8309   $194.83   $194.85   $194.80  
  Purchase #4 $126.67   13.144%   $143.3195   $143.32   $143.30   $143.30  
  Purchase #5 $54.58   4.580%   $57.0798   $57.08   $57.10   $57.10  
  Purchase #6 $103.71   6.474%   $110.4242   $110.42   $110.40   $110.40  
  Purchase #7 $62.35   13.189%   $70.5733   $70.57   $70.55   $70.60  
  7 purchase totals = $593.67       $644.8453   $644.84   $644.85   $644.80