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 $8.58   11.278%   $9.5477   $9.55   $9.55   $9.50  
  Purchase #2 $11.86   13.858%   $13.5036   $13.50   $13.50   $13.50  
  Purchase #3 $63.64   9.628%   $69.7673   $69.77   $69.75   $69.80  
  Purchase #4 $29.31   11.401%   $32.6516   $32.65   $32.65   $32.70  
  Purchase #5 $77.30   4.129%   $80.4917   $80.49   $80.50   $80.50  
  Purchase #6 $47.65   8.578%   $51.7374   $51.74   $51.75   $51.70  
  Purchase #7 $144.65   12.353%   $162.5186   $162.52   $162.50   $162.50  
  7 purchase totals = $382.99       $420.2179   $420.22   $420.20   $420.20