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 $17.52   12.712%   $19.7471   $19.75   $19.75   $19.70  
  Purchase #2 $37.72   3.422%   $39.0108   $39.01   $39.00   $39.00  
  Purchase #3 $35.94   9.728%   $39.4362   $39.44   $39.45   $39.40  
  Purchase #4 $169.66   8.292%   $183.7282   $183.73   $183.75   $183.70  
  Purchase #5 $130.25   7.542%   $140.0735   $140.07   $140.05   $140.10  
  Purchase #6 $131.87   5.283%   $138.8367   $138.84   $138.85   $138.80  
  Purchase #7 $80.77   6.830%   $86.2866   $86.29   $86.30   $86.30  
  7 purchase totals = $603.73       $647.1191   $647.13   $647.15   $647.00