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 $20.39   3.467%   $21.0969   $21.10   $21.10   $21.10  
  Purchase #2 $71.56   13.297%   $81.0753   $81.08   $81.10   $81.10  
  Purchase #3 $218.80   9.572%   $239.7435   $239.74   $239.75   $239.70  
  Purchase #4 $45.28   12.805%   $51.0781   $51.08   $51.10   $51.10  
  Purchase #5 $73.95   4.768%   $77.4759   $77.48   $77.50   $77.50  
  Purchase #6 $221.76   7.638%   $238.698   $238.70   $238.70   $238.70  
  Purchase #7 $185.75   8.318%   $201.2007   $201.20   $201.20   $201.20  
  7 purchase totals = $837.49       $910.3684   $910.38   $910.45   $910.40