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 $122.37   13.438%   $138.8141   $138.81   $138.80   $138.80  
  Purchase #2 $103.85   6.863%   $110.9772   $110.98   $111.00   $111.00  
  Purchase #3 $118.33   10.679%   $130.9665   $130.97   $130.95   $131.00  
  Purchase #4 $210.97   13.680%   $239.8307   $239.83   $239.85   $239.80  
  Purchase #5 $21.34   13.806%   $24.2862   $24.29   $24.30   $24.30  
  Purchase #6 $38.44   3.379%   $39.7389   $39.74   $39.75   $39.70  
  Purchase #7 $90.48   10.603%   $100.0736   $100.07   $100.05   $100.10  
  7 purchase totals = $705.78       $784.6872   $784.69   $784.70   $784.70