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 $155.80   12.824%   $175.7798   $175.78   $175.80   $175.80  
  Purchase #2 $143.49   11.413%   $159.8665   $159.87   $159.85   $159.90  
  Purchase #3 $107.35   11.170%   $119.341   $119.34   $119.35   $119.30  
  Purchase #4 $199.36   5.211%   $209.7486   $209.75   $209.75   $209.70  
  Purchase #5 $34.39   13.315%   $38.969   $38.97   $38.95   $39.00  
  Purchase #6 $156.87   7.274%   $168.2807   $168.28   $168.30   $168.30  
  Purchase #7 $65.51   5.786%   $69.3004   $69.30   $69.30   $69.30  
  7 purchase totals = $862.77       $941.2860   $941.29   $941.30   $941.30