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 $117.38   3.390%   $121.3592   $121.36   $121.35   $121.40  
  Purchase #2 $35.92   11.326%   $39.9883   $39.99   $40.00   $40.00  
  Purchase #3 $68.45   5.332%   $72.0998   $72.10   $72.10   $72.10  
  Purchase #4 $17.96   13.439%   $20.3736   $20.37   $20.35   $20.40  
  Purchase #5 $244.56   13.569%   $277.7443   $277.74   $277.75   $277.70  
  Purchase #6 $130.32   7.165%   $139.6574   $139.66   $139.65   $139.70  
  Purchase #7 $233.59   13.370%   $264.821   $264.82   $264.80   $264.80  
  7 purchase totals = $848.18       $936.0436   $936.04   $936.00   $936.10