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 $17.38   3.506%   $17.9893   $17.99   $18.00   $18.00  
  Purchase #2 $166.83   7.753%   $179.7643   $179.76   $179.75   $179.80  
  Purchase #3 $32.84   13.671%   $37.3296   $37.33   $37.35   $37.30  
  Purchase #4 $103.37   5.494%   $109.0491   $109.05   $109.05   $109.00  
  Purchase #5 $229.55   13.347%   $260.188   $260.19   $260.20   $260.20  
  Purchase #6 $247.57   10.820%   $274.3571   $274.36   $274.35   $274.40  
  Purchase #7 $38.80   10.111%   $42.7231   $42.72   $42.70   $42.70  
  7 purchase totals = $836.34       $921.4005   $921.40   $921.40   $921.40