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 $79.99   13.324%   $90.6479   $90.65   $90.65   $90.60  
  Purchase #2 $239.27   5.528%   $252.4968   $252.50   $252.50   $252.50  
  Purchase #3 $179.80   13.303%   $203.7188   $203.72   $203.70   $203.70  
  Purchase #4 $196.88   11.215%   $218.9601   $218.96   $218.95   $219.00  
  Purchase #5 $209.54   7.420%   $225.0879   $225.09   $225.10   $225.10  
  Purchase #6 $28.98   10.364%   $31.9835   $31.98   $32.00   $32.00  
  Purchase #7 $169.36   11.343%   $188.5705   $188.57   $188.55   $188.60  
  7 purchase totals = $1,103.82       $1,211.4655   $1,211.47   $1,211.45   $1,211.50