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 $28.71   9.479%   $31.4314   $31.43   $31.45   $31.40  
  Purchase #2 $190.43   12.829%   $214.8603   $214.86   $214.85   $214.90  
  Purchase #3 $144.26   3.125%   $148.7681   $148.77   $148.75   $148.80  
  Purchase #4 $132.74   13.181%   $150.2365   $150.24   $150.25   $150.20  
  Purchase #5 $9.85   4.107%   $10.2545   $10.25   $10.25   $10.30  
  Purchase #6 $110.28   6.605%   $117.564   $117.56   $117.55   $117.60  
  Purchase #7 $32.38   13.759%   $36.8352   $36.84   $36.85   $36.80  
  7 purchase totals = $648.65       $709.9500   $709.95   $709.95   $710.00