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 $172.78   10.533%   $190.9789   $190.98   $191.00   $191.00  
  Purchase #2 $29.53   5.745%   $31.2265   $31.23   $31.25   $31.20  
  Purchase #3 $122.30   5.655%   $129.2161   $129.22   $129.20   $129.20  
  Purchase #4 $138.53   10.512%   $153.0923   $153.09   $153.10   $153.10  
  Purchase #5 $8.54   12.411%   $9.5999   $9.60   $9.60   $9.60  
  Purchase #6 $27.74   9.258%   $30.3082   $30.31   $30.30   $30.30  
  Purchase #7 $74.67   13.674%   $84.8804   $84.88   $84.90   $84.90  
  7 purchase totals = $574.09       $629.3023   $629.31   $629.35   $629.30