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 $20.55   12.270%   $23.0715   $23.07   $23.05   $23.10  
  Purchase #2 $171.81   4.714%   $179.9091   $179.91   $179.90   $179.90  
  Purchase #3 $140.37   13.469%   $159.2764   $159.28   $159.30   $159.30  
  Purchase #4 $9.74   13.760%   $11.0802   $11.08   $11.10   $11.10  
  Purchase #5 $79.84   10.479%   $88.2064   $88.21   $88.20   $88.20  
  Purchase #6 $101.58   3.201%   $104.8316   $104.83   $104.85   $104.80  
  Purchase #7 $105.57   10.514%   $116.6696   $116.67   $116.65   $116.70  
  7 purchase totals = $629.46       $683.0448   $683.05   $683.05   $683.10