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 $8.92   9.169%   $9.7379   $9.74   $9.75   $9.70  
  Purchase #2 $8.43   7.448%   $9.0579   $9.06   $9.05   $9.10  
  Purchase #3 $28.26   4.563%   $29.5495   $29.55   $29.55   $29.50  
  Purchase #4 $18.47   6.759%   $19.7184   $19.72   $19.70   $19.70  
  Purchase #5 $200.41   11.761%   $223.9802   $223.98   $224.00   $224.00  
  Purchase #6 $29.86   12.835%   $33.6925   $33.69   $33.70   $33.70  
  Purchase #7 $189.89   12.355%   $213.3509   $213.35   $213.35   $213.40  
  7 purchase totals = $484.24       $539.0873   $539.09   $539.10   $539.10