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 $145.34   9.457%   $159.0848   $159.08   $159.10   $159.10  
  Purchase #2 $27.68   12.866%   $31.2413   $31.24   $31.25   $31.20  
  Purchase #3 $90.86   9.514%   $99.5044   $99.50   $99.50   $99.50  
  Purchase #4 $174.65   7.396%   $187.5671   $187.57   $187.55   $187.60  
  Purchase #5 $32.69   8.738%   $35.5465   $35.55   $35.55   $35.50  
  Purchase #6 $193.53   8.765%   $210.4929   $210.49   $210.50   $210.50  
  Purchase #7 $94.86   12.863%   $107.0618   $107.06   $107.05   $107.10  
  7 purchase totals = $759.61       $830.4988   $830.49   $830.50   $830.50