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 $9.32   5.254%   $9.8097   $9.81   $9.80   $9.80  
  Purchase #2 $79.38   11.761%   $88.7159   $88.72   $88.70   $88.70  
  Purchase #3 $31.96   4.563%   $33.4183   $33.42   $33.40   $33.40  
  Purchase #4 $28.43   12.216%   $31.903   $31.90   $31.90   $31.90  
  Purchase #5 $141.56   10.363%   $156.2299   $156.23   $156.25   $156.20  
  Purchase #6 $29.25   5.800%   $30.9465   $30.95   $30.95   $30.90  
  Purchase #7 $71.34   11.566%   $79.5912   $79.59   $79.60   $79.60  
  7 purchase totals = $391.24       $430.6145   $430.62   $430.60   $430.50