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 $34.78   12.389%   $39.0889   $39.09   $39.10   $39.10  
  Purchase #2 $12.94   6.628%   $13.7977   $13.80   $13.80   $13.80  
  Purchase #3 $148.31   8.850%   $161.4354   $161.44   $161.45   $161.40  
  Purchase #4 $7.78   5.118%   $8.1782   $8.18   $8.20   $8.20  
  Purchase #5 $220.35   10.536%   $243.5661   $243.57   $243.55   $243.60  
  Purchase #6 $170.39   9.501%   $186.5788   $186.58   $186.60   $186.60  
  Purchase #7 $44.48   9.202%   $48.573   $48.57   $48.55   $48.60  
  7 purchase totals = $639.03       $701.2181   $701.23   $701.25   $701.30