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 $235.62   8.643%   $255.9846   $255.98   $256.00   $256.00  
  Purchase #2 $140.52   8.473%   $152.4263   $152.43   $152.45   $152.40  
  Purchase #3 $61.39   11.311%   $68.3338   $68.33   $68.35   $68.30  
  Purchase #4 $236.96   13.251%   $268.3596   $268.36   $268.35   $268.40  
  Purchase #5 $224.99   5.502%   $237.3689   $237.37   $237.35   $237.40  
  Purchase #6 $31.48   6.628%   $33.5665   $33.57   $33.55   $33.60  
  Purchase #7 $172.84   12.363%   $194.2082   $194.21   $194.20   $194.20  
  7 purchase totals = $1,103.80       $1,210.2479   $1,210.25   $1,210.25   $1,210.30