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 $207.78   6.152%   $220.5626   $220.56   $220.55   $220.60  
  Purchase #2 $57.44   12.199%   $64.4471   $64.45   $64.45   $64.40  
  Purchase #3 $167.91   11.830%   $187.7738   $187.77   $187.75   $187.80  
  Purchase #4 $171.43   6.210%   $182.0758   $182.08   $182.10   $182.10  
  Purchase #5 $14.52   11.719%   $16.2216   $16.22   $16.20   $16.20  
  Purchase #6 $220.31   3.383%   $227.7631   $227.76   $227.75   $227.80  
  Purchase #7 $116.79   4.828%   $122.4286   $122.43   $122.45   $122.40  
  7 purchase totals = $956.18       $1,021.2726   $1,021.27   $1,021.25   $1,021.30