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 $31.53   8.446%   $34.193   $34.19   $34.20   $34.20  
  Purchase #2 $7.44   3.669%   $7.713   $7.71   $7.70   $7.70  
  Purchase #3 $62.53   7.102%   $66.9709   $66.97   $66.95   $67.00  
  Purchase #4 $196.84   6.613%   $209.857   $209.86   $209.85   $209.90  
  Purchase #5 $41.93   13.142%   $47.4404   $47.44   $47.45   $47.40  
  Purchase #6 $6.51   11.784%   $7.2771   $7.28   $7.30   $7.30  
  Purchase #7 $128.96   9.852%   $141.6651   $141.67   $141.65   $141.70  
  7 purchase totals = $475.74       $515.1165   $515.12   $515.10   $515.20