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 $80.90   13.239%   $91.6104   $91.61   $91.60   $91.60  
  Purchase #2 $28.39   13.576%   $32.2442   $32.24   $32.25   $32.20  
  Purchase #3 $194.83   12.171%   $218.5428   $218.54   $218.55   $218.50  
  Purchase #4 $197.50   11.671%   $220.5502   $220.55   $220.55   $220.60  
  Purchase #5 $183.79   5.163%   $193.2791   $193.28   $193.30   $193.30  
  Purchase #6 $24.73   11.426%   $27.5556   $27.56   $27.55   $27.60  
  Purchase #7 $131.48   4.614%   $137.5465   $137.55   $137.55   $137.50  
  7 purchase totals = $841.62       $921.3288   $921.33   $921.35   $921.30