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 $166.42   8.356%   $180.3261   $180.33   $180.35   $180.30  
  Purchase #2 $16.49   12.282%   $18.5153   $18.52   $18.50   $18.50  
  Purchase #3 $15.50   10.763%   $17.1683   $17.17   $17.15   $17.20  
  Purchase #4 $8.36   13.327%   $9.4741   $9.47   $9.45   $9.50  
  Purchase #5 $20.62   13.691%   $23.4431   $23.44   $23.45   $23.40  
  Purchase #6 $103.46   13.611%   $117.5419   $117.54   $117.55   $117.50  
  Purchase #7 $193.88   9.380%   $212.0659   $212.07   $212.05   $212.10  
  7 purchase totals = $524.73       $578.5347   $578.54   $578.50   $578.50