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   6.483%   $21.8823   $21.88   $21.90   $21.90  
  Purchase #2 $12.55   3.839%   $13.0318   $13.03   $13.05   $13.00  
  Purchase #3 $201.45   10.759%   $223.124   $223.12   $223.10   $223.10  
  Purchase #4 $9.79   10.244%   $10.7929   $10.79   $10.80   $10.80  
  Purchase #5 $174.47   5.672%   $184.3659   $184.37   $184.35   $184.40  
  Purchase #6 $206.41   12.735%   $232.6963   $232.70   $232.70   $232.70  
  Purchase #7 $226.48   12.799%   $255.4672   $255.47   $255.45   $255.50  
  7 purchase totals = $851.70       $941.3604   $941.36   $941.35   $941.40