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 $189.35   4.841%   $198.5164   $198.52   $198.50   $198.50  
  Purchase #2 $243.99   13.793%   $277.6435   $277.64   $277.65   $277.60  
  Purchase #3 $197.41   8.434%   $214.0596   $214.06   $214.05   $214.10  
  Purchase #4 $235.38   9.523%   $257.7952   $257.80   $257.80   $257.80  
  Purchase #5 $214.94   6.131%   $228.118   $228.12   $228.10   $228.10  
  Purchase #6 $247.74   13.707%   $281.6977   $281.70   $281.70   $281.70  
  Purchase #7 $119.32   11.674%   $133.2494   $133.25   $133.25   $133.20  
  7 purchase totals = $1,448.13       $1,591.0798   $1,591.09   $1,591.05   $1,591.00