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 $120.47   5.716%   $127.3561   $127.36   $127.35   $127.40  
  Purchase #2 $78.88   11.118%   $87.6499   $87.65   $87.65   $87.60  
  Purchase #3 $239.79   11.741%   $267.9437   $267.94   $267.95   $267.90  
  Purchase #4 $206.31   4.159%   $214.8904   $214.89   $214.90   $214.90  
  Purchase #5 $232.96   4.310%   $243.0006   $243.00   $243.00   $243.00  
  Purchase #6 $52.28   9.515%   $57.2544   $57.25   $57.25   $57.30  
  Purchase #7 $26.27   11.240%   $29.2227   $29.22   $29.20   $29.20  
  7 purchase totals = $956.96       $1,027.3178   $1,027.31   $1,027.30   $1,027.30