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 $227.34   12.176%   $255.0209   $255.02   $255.00   $255.00  
  Purchase #2 $247.36   10.282%   $272.7936   $272.79   $272.80   $272.80  
  Purchase #3 $123.87   4.198%   $129.0701   $129.07   $129.05   $129.10  
  Purchase #4 $88.98   8.289%   $96.3556   $96.36   $96.35   $96.40  
  Purchase #5 $6.82   12.227%   $7.6539   $7.65   $7.65   $7.70  
  Purchase #6 $195.77   10.477%   $216.2808   $216.28   $216.30   $216.30  
  Purchase #7 $23.26   6.500%   $24.7719   $24.77   $24.75   $24.80  
  7 purchase totals = $913.40       $1,001.9468   $1,001.94   $1,001.90   $1,002.10