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 $105.56   3.324%   $109.0688   $109.07   $109.05   $109.10  
  Purchase #2 $28.98   13.302%   $32.8349   $32.83   $32.85   $32.80  
  Purchase #3 $152.26   7.214%   $163.244   $163.24   $163.25   $163.20  
  Purchase #4 $138.74   10.780%   $153.6962   $153.70   $153.70   $153.70  
  Purchase #5 $25.77   11.433%   $28.7163   $28.72   $28.70   $28.70  
  Purchase #6 $37.76   11.824%   $42.2247   $42.22   $42.20   $42.20  
  Purchase #7 $197.51   12.150%   $221.5075   $221.51   $221.50   $221.50  
  7 purchase totals = $686.58       $751.2924   $751.29   $751.25   $751.20