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 $58.68   10.806%   $65.021   $65.02   $65.00   $65.00  
  Purchase #2 $12.41   6.528%   $13.2201   $13.22   $13.20   $13.20  
  Purchase #3 $214.71   12.283%   $241.0828   $241.08   $241.10   $241.10  
  Purchase #4 $240.74   13.514%   $273.2736   $273.27   $273.25   $273.30  
  Purchase #5 $78.58   4.744%   $82.3078   $82.31   $82.30   $82.30  
  Purchase #6 $49.99   4.862%   $52.4205   $52.42   $52.40   $52.40  
  Purchase #7 $133.38   4.606%   $139.5235   $139.52   $139.50   $139.50  
  7 purchase totals = $788.49       $866.8493   $866.84   $866.75   $866.80