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 $28.31   8.679%   $30.767   $30.77   $30.75   $30.80  
  Purchase #2 $15.59   6.731%   $16.6394   $16.64   $16.65   $16.60  
  Purchase #3 $174.29   9.721%   $191.2327   $191.23   $191.25   $191.20  
  Purchase #4 $88.70   11.705%   $99.0823   $99.08   $99.10   $99.10  
  Purchase #5 $87.90   10.867%   $97.4521   $97.45   $97.45   $97.50  
  Purchase #6 $125.64   10.679%   $139.0571   $139.06   $139.05   $139.10  
  Purchase #7 $65.62   8.398%   $71.1308   $71.13   $71.15   $71.10  
  7 purchase totals = $586.05       $645.3614   $645.36   $645.40   $645.40