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 $29.36   5.858%   $31.0799   $31.08   $31.10   $31.10  
  Purchase #2 $89.32   9.374%   $97.6929   $97.69   $97.70   $97.70  
  Purchase #3 $37.71   3.383%   $38.9857   $38.99   $39.00   $39.00  
  Purchase #4 $164.37   6.529%   $175.1017   $175.10   $175.10   $175.10  
  Purchase #5 $45.58   7.755%   $49.1147   $49.11   $49.10   $49.10  
  Purchase #6 $175.51   7.281%   $188.2889   $188.29   $188.30   $188.30  
  Purchase #7 $85.96   4.733%   $90.0285   $90.03   $90.05   $90.00  
  7 purchase totals = $627.81       $670.2923   $670.29   $670.35   $670.30