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 $29.99   11.452%   $33.4245   $33.42   $33.40   $33.40  
  Purchase #2 $62.25   7.104%   $66.6722   $66.67   $66.65   $66.70  
  Purchase #3 $222.28   8.296%   $240.7203   $240.72   $240.70   $240.70  
  Purchase #4 $82.57   5.112%   $86.791   $86.79   $86.80   $86.80  
  Purchase #5 $183.29   12.204%   $205.6587   $205.66   $205.65   $205.70  
  Purchase #6 $181.46   5.421%   $191.2969   $191.30   $191.30   $191.30  
  Purchase #7 $186.78   4.438%   $195.0693   $195.07   $195.05   $195.10  
  7 purchase totals = $948.62       $1,019.6329   $1,019.63   $1,019.55   $1,019.70