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 $189.74   7.717%   $204.3822   $204.38   $204.40   $204.40  
  Purchase #2 $27.47   7.300%   $29.4753   $29.48   $29.50   $29.50  
  Purchase #3 $94.69   9.283%   $103.4801   $103.48   $103.50   $103.50  
  Purchase #4 $146.61   8.470%   $159.0279   $159.03   $159.05   $159.00  
  Purchase #5 $127.98   4.515%   $133.7583   $133.76   $133.75   $133.80  
  Purchase #6 $34.75   9.740%   $38.1347   $38.13   $38.15   $38.10  
  Purchase #7 $104.63   5.585%   $110.4736   $110.47   $110.45   $110.50  
  7 purchase totals = $725.87       $778.7321   $778.73   $778.80   $778.80