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 $5.99   13.390%   $6.7921   $6.79   $6.80   $6.80  
  Purchase #2 $39.56   8.738%   $43.0168   $43.02   $43.00   $43.00  
  Purchase #3 $13.96   12.694%   $15.7321   $15.73   $15.75   $15.70  
  Purchase #4 $133.71   11.868%   $149.5787   $149.58   $149.60   $149.60  
  Purchase #5 $115.56   7.717%   $124.4778   $124.48   $124.50   $124.50  
  Purchase #6 $24.74   12.398%   $27.8073   $27.81   $27.80   $27.80  
  Purchase #7 $105.89   4.214%   $110.3522   $110.35   $110.35   $110.40  
  7 purchase totals = $439.41       $477.7570   $477.76   $477.80   $477.80