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 $100.67   3.709%   $104.4039   $104.40   $104.40   $104.40  
  Purchase #2 $29.99   4.202%   $31.2502   $31.25   $31.25   $31.30  
  Purchase #3 $131.27   6.440%   $139.7238   $139.72   $139.70   $139.70  
  Purchase #4 $99.65   5.444%   $105.0749   $105.07   $105.05   $105.10  
  Purchase #5 $97.25   7.133%   $104.1868   $104.19   $104.20   $104.20  
  Purchase #6 $9.57   5.663%   $10.1119   $10.11   $10.10   $10.10  
  Purchase #7 $48.31   8.447%   $52.3907   $52.39   $52.40   $52.40  
  7 purchase totals = $516.71       $547.1422   $547.13   $547.10   $547.20