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 $150.97   8.765%   $164.2025   $164.20   $164.20   $164.20  
  Purchase #2 $111.62   11.475%   $124.4284   $124.43   $124.45   $124.40  
  Purchase #3 $155.89   11.305%   $173.5134   $173.51   $173.50   $173.50  
  Purchase #4 $67.38   7.850%   $72.6693   $72.67   $72.65   $72.70  
  Purchase #5 $123.60   10.492%   $136.5681   $136.57   $136.55   $136.60  
  Purchase #6 $76.35   5.471%   $80.5271   $80.53   $80.55   $80.50  
  Purchase #7 $184.54   12.761%   $208.0891   $208.09   $208.10   $208.10  
  7 purchase totals = $870.35       $959.9979   $960.00   $960.00   $960.00