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 $7.69   12.783%   $8.673   $8.67   $8.65   $8.70  
  Purchase #2 $95.70   13.865%   $108.9688   $108.97   $108.95   $109.00  
  Purchase #3 $245.54   8.436%   $266.2538   $266.25   $266.25   $266.30  
  Purchase #4 $177.56   9.628%   $194.6555   $194.66   $194.65   $194.70  
  Purchase #5 $104.88   8.191%   $113.4707   $113.47   $113.45   $113.50  
  Purchase #6 $140.30   3.481%   $145.1838   $145.18   $145.20   $145.20  
  Purchase #7 $227.95   11.286%   $253.6764   $253.68   $253.70   $253.70  
  7 purchase totals = $999.62       $1,090.8820   $1,090.88   $1,090.85   $1,091.10