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 $16.27   12.495%   $18.3029   $18.30   $18.30   $18.30  
  Purchase #2 $107.44   7.352%   $115.339   $115.34   $115.35   $115.30  
  Purchase #3 $88.79   8.272%   $96.1347   $96.13   $96.15   $96.10  
  Purchase #4 $121.94   7.281%   $130.8185   $130.82   $130.80   $130.80  
  Purchase #5 $31.33   6.780%   $33.4542   $33.45   $33.45   $33.50  
  Purchase #6 $19.44   7.205%   $20.8407   $20.84   $20.85   $20.80  
  Purchase #7 $139.82   7.759%   $150.6686   $150.67   $150.65   $150.70  
  7 purchase totals = $525.03       $565.5586   $565.55   $565.55   $565.50