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 $131.89   10.255%   $145.4153   $145.42   $145.40   $145.40  
  Purchase #2 $19.37   13.326%   $21.9512   $21.95   $21.95   $22.00  
  Purchase #3 $127.30   12.850%   $143.6581   $143.66   $143.65   $143.70  
  Purchase #4 $34.45   9.656%   $37.7765   $37.78   $37.80   $37.80  
  Purchase #5 $59.35   5.454%   $62.5869   $62.59   $62.60   $62.60  
  Purchase #6 $188.37   7.531%   $202.5561   $202.56   $202.55   $202.60  
  Purchase #7 $21.30   12.371%   $23.935   $23.94   $23.95   $23.90  
  7 purchase totals = $582.03       $637.8791   $637.90   $637.90   $638.00