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 $124.40   3.688%   $128.9879   $128.99   $129.00   $129.00  
  Purchase #2 $86.25   4.484%   $90.1175   $90.12   $90.10   $90.10  
  Purchase #3 $129.33   8.289%   $140.0502   $140.05   $140.05   $140.10  
  Purchase #4 $127.87   8.192%   $138.3451   $138.35   $138.35   $138.30  
  Purchase #5 $158.94   8.746%   $172.8409   $172.84   $172.85   $172.80  
  Purchase #6 $165.97   13.684%   $188.6813   $188.68   $188.70   $188.70  
  Purchase #7 $80.91   6.778%   $86.3941   $86.39   $86.40   $86.40  
  7 purchase totals = $873.67       $945.4170   $945.42   $945.45   $945.40