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 $238.76   7.762%   $257.2926   $257.29   $257.30   $257.30  
  Purchase #2 $82.79   4.775%   $86.7432   $86.74   $86.75   $86.70  
  Purchase #3 $201.51   13.337%   $228.3854   $228.39   $228.40   $228.40  
  Purchase #4 $97.31   4.399%   $101.5907   $101.59   $101.60   $101.60  
  Purchase #5 $118.50   6.615%   $126.3388   $126.34   $126.35   $126.30  
  Purchase #6 $167.61   10.278%   $184.837   $184.84   $184.85   $184.80  
  Purchase #7 $49.59   9.842%   $54.4706   $54.47   $54.45   $54.50  
  7 purchase totals = $956.07       $1,039.6583   $1,039.66   $1,039.70   $1,039.60