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 $119.98   13.252%   $135.8797   $135.88   $135.90   $135.90  
  Purchase #2 $123.68   8.341%   $133.9961   $134.00   $134.00   $134.00  
  Purchase #3 $112.38   3.566%   $116.3875   $116.39   $116.40   $116.40  
  Purchase #4 $31.98   12.571%   $36.0002   $36.00   $36.00   $36.00  
  Purchase #5 $180.79   10.644%   $200.0333   $200.03   $200.05   $200.00  
  Purchase #6 $200.52   7.538%   $215.6352   $215.64   $215.65   $215.60  
  Purchase #7 $198.74   7.457%   $213.56   $213.56   $213.55   $213.60  
  7 purchase totals = $968.07       $1,051.4920   $1,051.50   $1,051.55   $1,051.50