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 $39.66   6.630%   $42.2895   $42.29   $42.30   $42.30  
  Purchase #2 $53.64   7.383%   $57.6002   $57.60   $57.60   $57.60  
  Purchase #3 $102.54   13.145%   $116.0189   $116.02   $116.00   $116.00  
  Purchase #4 $225.41   9.434%   $246.6752   $246.68   $246.70   $246.70  
  Purchase #5 $246.25   11.451%   $274.4481   $274.45   $274.45   $274.40  
  Purchase #6 $54.98   4.797%   $57.6174   $57.62   $57.60   $57.60  
  Purchase #7 $246.33   5.670%   $260.2969   $260.30   $260.30   $260.30  
  7 purchase totals = $968.81       $1,054.9462   $1,054.96   $1,054.95   $1,054.90