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 $134.60   11.378%   $149.9148   $149.91   $149.90   $149.90  
  Purchase #2 $18.58   7.519%   $19.977   $19.98   $20.00   $20.00  
  Purchase #3 $144.72   13.856%   $164.7724   $164.77   $164.75   $164.80  
  Purchase #4 $143.27   9.522%   $156.9122   $156.91   $156.90   $156.90  
  Purchase #5 $163.57   9.595%   $179.2645   $179.26   $179.25   $179.30  
  Purchase #6 $38.71   3.200%   $39.9487   $39.95   $39.95   $39.90  
  Purchase #7 $209.45   5.388%   $220.7352   $220.74   $220.75   $220.70  
  7 purchase totals = $852.90       $931.5248   $931.52   $931.50   $931.50