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 $139.46   8.359%   $151.1175   $151.12   $151.10   $151.10  
  Purchase #2 $156.96   6.293%   $166.8375   $166.84   $166.85   $166.80  
  Purchase #3 $85.32   11.682%   $95.2871   $95.29   $95.30   $95.30  
  Purchase #4 $175.47   12.527%   $197.4511   $197.45   $197.45   $197.50  
  Purchase #5 $33.81   11.524%   $37.7063   $37.71   $37.70   $37.70  
  Purchase #6 $243.83   11.298%   $271.3779   $271.38   $271.40   $271.40  
  Purchase #7 $47.43   5.852%   $50.2056   $50.21   $50.20   $50.20  
  7 purchase totals = $882.28       $969.9830   $970.00   $970.00   $970.00